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                                      IPv4

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   Internet Protocol version 4 (IPv4) is the fourth version of the Internet
   Protocol (IP). It is one of the core protocols of standards-based
   internetworking methods in the Internet and other packet-switched
   networks. IPv4 was the first version deployed for production on SATNET in
   1982 and on the ARPANET in January 1983. It is still used to route most
   Internet traffic today,^[1] even with the ongoing deployment of Internet
   Protocol version 6 (IPv6),^[2] its successor.

                          Internet Protocol version 4
   Protocol stack
   IPv4 Packet-en.svg
   IPv4 packet  
   Purpose      internetworking protocol 
   Developer(s) DARPA                    
   Introduction 1981; 41 years ago       
   OSI layer    Network layer            
   RFC(s)       791                      

   IPv4 uses a 32-bit address space which provides 4,294,967,296 (2^32)
   unique addresses, but large blocks are reserved for special networking
   purposes.^[3]^[4]

Contents

     * 1 History
     * 2 Purpose
     * 3 Addressing
          * 3.1 Address representations
          * 3.2 Allocation
          * 3.3 Special-use addresses
               * 3.3.1 Private networks
          * 3.4 Link-local addressing
          * 3.5 Loopback
          * 3.6 First and last subnet addresses
          * 3.7 Address resolution
          * 3.8 Unnumbered interface
     * 4 Address space exhaustion
     * 5 Packet structure
          * 5.1 Header
          * 5.2 Data
     * 6 Fragmentation and reassembly
          * 6.1 Fragmentation
          * 6.2 Reassembly
     * 7 Assistive protocols
     * 8 See also
     * 9 Notes
     * 10 References
     * 11 External links

HistoryEdit

   Internet Protocol version 4 is described in IETF publication RFC 791
   (September 1981), replacing an earlier definition of January 1980 (RFC
   760). In March 1982, the US Department of Defense decided on the Internet
   Protocol Suite (TCP/IP) as the standard for all military computer
   networking.^[5]

PurposeEdit

   The Internet Protocol is the protocol that defines and enables
   internetworking at the internet layer of the Internet Protocol Suite. In
   essence it forms the Internet. It uses a logical addressing system and
   performs routing, which is the forwarding of packets from a source host to
   the next router that is one hop closer to the intended destination host on
   another network.

   IPv4 is a connectionless protocol, and operates on a best-effort delivery
   model, in that it does not guarantee delivery, nor does it assure proper
   sequencing or avoidance of duplicate delivery. These aspects, including
   data integrity, are addressed by an upper layer transport protocol, such
   as the Transmission Control Protocol (TCP).

AddressingEdit

   [IMG] 
   Enlarge
   Decomposition of the quad-dotted IPv4 address representation to its binary
   value

   IPv4 uses 32-bit addresses which limits the address space to 4294967296
   (2^32) addresses.

   IPv4 reserves special address blocks for private networks (~18 million
   addresses) and multicast addresses (~270 million addresses).

  Address representationsEdit

   IPv4 addresses may be represented in any notation expressing a 32-bit
   integer value. They are most often written in dot-decimal notation, which
   consists of four octets of the address expressed individually in decimal
   numbers and separated by periods.

   For example, the quad-dotted IP address 192.0.2.235 represents the 32-bit
   decimal number 3221226219, which in hexadecimal format is 0xC00002EB.

   CIDR notation combines the address with its routing prefix in a compact
   format, in which the address is followed by a slash character (/) and the
   count of leading consecutive 1 bits in the routing prefix (subnet mask).

   Other address representations were in common use when classful networking
   was practiced. For example, the loopback address 127.0.0.1 is commonly
   written as 127.1, given that it belongs to a class-A network with eight
   bits for the network mask and 24 bits for the host number. When fewer than
   four numbers are specified in the address in dotted notation, the last
   value is treated as an integer of as many bytes as are required to fill
   out the address to four octets. Thus, the address 127.65530 is equivalent
   to 127.0.255.250.

  AllocationEdit

   In the original design of IPv4, an IP address was divided into two parts:
   the network identifier was the most significant octet of the address, and
   the host identifier was the rest of the address. The latter was also
   called the rest field. This structure permitted a maximum of 256 network
   identifiers, which was quickly found to be inadequate.

   To overcome this limit, the most-significant address octet was redefined
   in 1981 to create network classes, in a system which later became known as
   classful networking. The revised system defined five classes. Classes A,
   B, and C had different bit lengths for network identification. The rest of
   the address was used as previously to identify a host within a network.
   Because of the different sizes of fields in different classes, each
   network class had a different capacity for addressing hosts. In addition
   to the three classes for addressing hosts, Class D was defined for
   multicast addressing and Class E was reserved for future applications.

   Dividing existing classful networks into subnets began in 1985 with the
   publication of
   Link: mw-deduplicated-inline-style
   RFC 950. This division was made more flexible with the introduction of
   variable-length subnet masks (VLSM) in
   Link: mw-deduplicated-inline-style
   RFC 1109 in 1987. In 1993, based on this work,
   Link: mw-deduplicated-inline-style
   RFC 1517 introduced Classless Inter-Domain Routing (CIDR),^[6] which
   expressed the number of bits (from the most significant) as, for instance,
   /24, and the class-based scheme was dubbed classful, by contrast. CIDR was
   designed to permit repartitioning of any address space so that smaller or
   larger blocks of addresses could be allocated to users. The hierarchical
   structure created by CIDR is managed by the Internet Assigned Numbers
   Authority (IANA) and the regional Internet registries (RIRs). Each RIR
   maintains a publicly searchable WHOIS database that provides information
   about IP address assignments.

  Special-use addressesEdit

   The Internet Engineering Task Force (IETF) and IANA have restricted from
   general use various reserved IP addresses for special purposes.^[7]
   Notably these addresses are used for multicast traffic and to provide
   addressing space for unrestricted uses on private networks.

                                 Special address blocks
Address block      Address range               Number of Scope           Description         
                                               addresses 
0.0.0.0/8          0.0.0.0–0.255.255.255        16777216 Software        Current network^[8] 
                                                                         Used for local      
10.0.0.0/8         10.0.0.0–10.255.255.255      16777216 Private network communications      
                                                                         within a private    
                                                                         network.^[9]        
                                                                         Shared address      
                                                                         space^[10] for      
                                                                         communications      
100.64.0.0/10      100.64.0.0–100.127.255.255    4194304 Private network between a service   
                                                                         provider and its    
                                                                         subscribers when    
                                                                         using a             
                                                                         carrier-grade NAT.  
                                                                         Used for loopback   
127.0.0.0/8        127.0.0.0–127.255.255.255    16777216 Host            addresses to the    
                                                                         local host.^[8]     
                                                                         Used for link-local 
                                                                         addresses^[11]      
                                                                         between two hosts   
                                                                         on a single link    
169.254.0.0/16     169.254.0.0–169.254.255.255     65536 Subnet          when no IP address  
                                                                         is otherwise        
                                                                         specified, such as  
                                                                         would have normally 
                                                                         been retrieved from 
                                                                         a DHCP server.      
                                                                         Used for local      
172.16.0.0/12      172.16.0.0–172.31.255.255     1048576 Private network communications      
                                                                         within a private    
                                                                         network.^[9]        
192.0.0.0/24       192.0.0.0–192.0.0.255             256 Private network IETF Protocol       
                                                                         Assignments.^[8]    
                                                                         Assigned as         
192.0.2.0/24       192.0.2.0–192.0.2.255             256 Documentation   TEST-NET-1,         
                                                                         documentation and   
                                                                         examples.^[12]      
                                                                         Reserved.^[13]      
                                                                         Formerly used for   
                                                                         IPv6 to IPv4        
192.88.99.0/24     192.88.99.0–192.88.99.255         256 Internet        relay^[14]          
                                                                         (included IPv6      
                                                                         address block       
                                                                         2002::/16).         
                                                                         Used for local      
192.168.0.0/16     192.168.0.0–192.168.255.255     65536 Private network communications      
                                                                         within a private    
                                                                         network.^[9]        
                                                                         Used for benchmark  
                                                                         testing of          
                                                                         inter-network       
198.18.0.0/15      198.18.0.0–198.19.255.255      131072 Private network communications      
                                                                         between two         
                                                                         separate            
                                                                         subnets.^[15]       
                                                                         Assigned as         
198.51.100.0/24    198.51.100.0–198.51.100.255       256 Documentation   TEST-NET-2,         
                                                                         documentation and   
                                                                         examples.^[12]      
                                                                         Assigned as         
203.0.113.0/24     203.0.113.0–203.0.113.255         256 Documentation   TEST-NET-3,         
                                                                         documentation and   
                                                                         examples.^[12]      
                                                                         In use for IP       
224.0.0.0/4        224.0.0.0–239.255.255.255   268435456 Internet        multicast.^[16]     
                                                                         (Former Class D     
                                                                         network.)           
                                                                         Assigned as         
233.252.0.0/24     233.252.0.0-233.252.0.255         256 Documentation   MCAST-TEST-NET,     
                                                                         documentation and   
                                                                         examples.^[16]^[17] 
                                                                         Reserved for future 
240.0.0.0/4        240.0.0.0–255.255.255.254   268435455 Internet        use.^[18] (Former   
                                                                         Class E network.)   
                                                                         Reserved for the    
255.255.255.255/32 255.255.255.255                     1 Subnet          "limited broadcast" 
                                                                         destination         
                                                                         address.^[8]^[19]   

    Private networksEdit

   Of the approximately four billion addresses defined in IPv4, about 18
   million addresses in three ranges are reserved for use in private
   networks. Packets addresses in these ranges are not routable in the public
   Internet; they are ignored by all public routers. Therefore, private hosts
   cannot directly communicate with public networks, but require network
   address translation at a routing gateway for this purpose.

                        Reserved private IPv4 network ranges^[9]
           Name   CIDR block     Address range   Number of Classful           
                                                 addresses description        
           24-bit 10.0.0.0/8     10.0.0.0 –       16777216 Single Class A.    
           block                 10.255.255.255  
           20-bit                172.16.0.0 –              Contiguous range   
           block  172.16.0.0/12  172.31.255.255    1048576 of 16 Class B      
                                                           blocks.            
           16-bit                192.168.0.0 –             Contiguous range   
           block  192.168.0.0/16 192.168.255.255     65536 of 256 Class C     
                                                           blocks.            

   Since two private networks, e.g., two branch offices, cannot directly
   interoperate via the public Internet, the two networks must be bridged
   across the Internet via a virtual private network (VPN) or an IP tunnel,
   which encapsulates packets, including their headers containing the private
   addresses, in a protocol layer during transmission across the public
   network. Additionally, encapsulated packets may be encrypted for
   transmission across public networks to secure the data.

  Link-local addressingEdit

   RFC 3927 defines the special address block 169.254.0.0/16 for link-local
   addressing. These addresses are only valid on the link (such as a local
   network segment or point-to-point connection) directly connected to a host
   that uses them. These addresses are not routable. Like private addresses,
   these addresses cannot be the source or destination of packets traversing
   the internet. These addresses are primarily used for address
   autoconfiguration (Zeroconf) when a host cannot obtain an IP address from
   a DHCP server or other internal configuration methods.

   When the address block was reserved, no standards existed for address
   autoconfiguration. Microsoft created an implementation called Automatic
   Private IP Addressing (APIPA), which was deployed on millions of machines
   and became a de facto standard. Many years later, in May 2005, the IETF
   defined a formal standard in RFC 3927, entitled Dynamic Configuration of
   IPv4 Link-Local Addresses.

  LoopbackEdit

   Main article: Localhost

   The class A network 127.0.0.0 (classless network 127.0.0.0/8) is reserved
   for loopback. IP packets whose source addresses belong to this network
   should never appear outside a host. Packets received on a non-loopback
   interface with a loopback source or destination address must be dropped.

  First and last subnet addressesEdit

   Link: mw-deduplicated-inline-style
   See also: IPv4 subnetting reference

   The first address in a subnet is used to identify the subnet itself. In
   this address all host bits are 0. To avoid ambiguity in representation,
   this address is reserved.^[20] The last address has all host bits set to
   1. It is used as a local broadcast address for sending messages to all
   devices on the subnet simultaneously. For networks of size /24 or larger,
   the broadcast address always ends in 255.

   For example, in the subnet 192.168.5.0/24 (subnet mask 255.255.255.0) the
   identifier 192.168.5.0 is used to refer to the entire subnet. The
   broadcast address of the network is 192.168.5.255.

                    Binary form                               Dot-decimal     
                                                              notation        
   Network space    11000000.10101000.00000101.00000000       192.168.5.0     
   Broadcast        11000000.10101000.00000101.11111111       192.168.5.255   
   address          
   In red, is shown the host part of the IP address; the other part is the
   network prefix. The host gets inverted (logical NOT), but the network
   prefix remains intact.

   However, this does not mean that every address ending in 0 or 255 cannot
   be used as a host address. For example, in the /16 subnet
   192.168.0.0/255.255.0.0, which is equivalent to the address range
   192.168.0.0–192.168.255.255, the broadcast address is 192.168.255.255. One
   can use the following addresses for hosts, even though they end with 255:
   192.168.1.255, 192.168.2.255, etc. Also, 192.168.0.0 is the network
   identifier and must not be assigned to an interface.^[21] The addresses
   192.168.1.0, 192.168.2.0, etc., may be assigned, despite ending with 0.

   In the past, conflict between network addresses and broadcast addresses
   arose because some software used non-standard broadcast addresses with
   zeros instead of ones.^[22]

   In networks smaller than /24, broadcast addresses do not necessarily end
   with 255. For example, a CIDR subnet 203.0.113.16/28 has the broadcast
   address 203.0.113.31.

                    Binary form                               Dot-decimal     
                                                              notation        
   Network space    11001011.00000000.01110001.00010000       203.0.113.16    
   Broadcast        11001011.00000000.01110001.00011111       203.0.113.31    
   address          
   In red, is shown the host part of the IP address; the other part is the
   network prefix. The host gets inverted (logical NOT), but the network
   prefix remains intact.

   As a special case, a /31 network has capacity for just two hosts. These
   networks are typically used for point-to-point connections. There is no
   network identifier or broadcast address for these networks.^[23]

  Address resolutionEdit

   Link: mw-deduplicated-inline-style
   Main article: Domain Name System

   Hosts on the Internet are usually known by names, e.g., www.example.com,
   not primarily by their IP address, which is used for routing and network
   interface identification. The use of domain names requires translating,
   called resolving, them to addresses and vice versa. This is analogous to
   looking up a phone number in a phone book using the recipient's name.

   The translation between addresses and domain names is performed by the
   Domain Name System (DNS), a hierarchical, distributed naming system that
   allows for the subdelegation of namespaces to other DNS servers.

  Unnumbered interfaceEdit

   A unnumbered point-to-point (PtP) link, also called a transit link, is a
   link that doesn't have any IP network or subnet number associated with it,
   but still have a IP address. First introduced in
   1993.^[24]^[25]^[26]^[27]^[28] The only purposes of a transit link is to
   route datagrams.

   Unnumbered link is used to free IP addresses, when having a scarce IP
   address space, or reduce the management of assigning IP and configuration
   of interfaces. Previous every link needs to dedicated /30 or /31 subnet
   using 2-4 IP addresses per point to point link. When a link is unnumbered
   a router-id is used, router-id is IP address /32 borrowed from a defined
   (normally a loopback) interface. The same router-id can be used on
   multiple interfaces.

   One of the disadvantage to unnumbered interface, is that is harder to do
   remote testing and management.

   Phil Karn from Qualcomm is credit as original designer.

Address space exhaustionEdit

   Link: mw-deduplicated-inline-style
   Main article: IPv4 address exhaustion

   Since the 1980s, it was apparent that the pool of available IPv4 addresses
   was depleting at a rate that was not initially anticipated in the original
   design of the network.^[29] The main market forces that accelerated
   address depletion included the rapidly growing number of Internet users,
   who increasingly used mobile computing devices, such as laptop computers,
   personal digital assistants (PDAs), and smart phones with IP data
   services. In addition, high-speed Internet access was based on always-on
   devices. The threat of exhaustion motivated the introduction of a number
   of remedial technologies, such as:

     * Classless Inter-Domain Routing (CIDR), for smaller ISP allocations
     * Unnumbered interface, removed the need on transit links.
     * network address translation, removed the need for end-to-end
       principle.

   By the mid-1990s, pervasive use of network address translation (NAT) in
   network access provider systems, and strict usage-based allocation
   policies at the regional and local Internet registries.

   The primary address pool of the Internet, maintained by IANA, was
   exhausted on 3 February 2011, when the last five blocks were allocated to
   the five RIRs.^[30]^[31] APNIC was the first RIR to exhaust its regional
   pool on 15 April 2011, except for a small amount of address space reserved
   for the transition technologies to IPv6, which is to be allocated under a
   restricted policy.^[32]

   The long-term solution to address exhaustion was the 1998 specification of
   a new version of the Internet Protocol, IPv6.^[33] It provides a vastly
   increased address space, but also allows improved route aggregation across
   the Internet, and offers large subnetwork allocations of a minimum of 2^64
   host addresses to end users. However, IPv4 is not directly interoperable
   with IPv6, so that IPv4-only hosts cannot directly communicate with
   IPv6-only hosts. With the phase-out of the 6bone experimental network
   starting in 2004, permanent formal deployment of IPv6 commenced in
   2006.^[34] Completion of IPv6 deployment is expected to take considerable
   time,^[35] so that intermediate transition technologies are necessary to
   permit hosts to participate in the Internet using both versions of the
   protocol.

Packet structureEdit

   An IP packet consists of a header section and a data section. An IP packet
   has no data checksum or any other footer after the data section. Typically
   the link layer encapsulates IP packets in frames with a CRC footer that
   detects most errors, many transport-layer protocols carried by IP also
   have their own error checking.^[36]

  HeaderEdit

   The IPv4 packet header consists of 14 fields, of which 13 are required.
   The 14th field is optional and aptly named: options. The fields in the
   header are packed with the most significant byte first (big endian), and
   for the diagram and discussion, the most significant bits are considered
   to come first (MSB 0 bit numbering). The most significant bit is numbered
   0, so the version field is actually found in the four most significant
   bits of the first byte, for example.

                               IPv4 header format
Offsets Octet 0               1                     2                       3  
Octet   Bit   0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 
0       0     Version IHL     DSCP            ECN   Total Length
4       32    Identification                        Flags    Fragment Offset
8       64    Time To Live    Protocol              Header Checksum
12      96    Source IP Address
16      128   Destination IP Address
20      160     
⋮       ⋮     Options (if IHL > 5)
56      448   

   Version
           The first header field in an IP packet is the four-bit version
           field. For IPv4, this is always equal to 4.

   Link: mw-deduplicated-inline-style
   Internet Header Length (IHL)
           The IPv4 header is variable in size due to the optional 14th field
           (options). The IHL field contains the size of the IPv4 header, it
           has 4 bits that specify the number of 32-bit words in the header.
           The minimum value for this field is 5,^[37] which indicates a
           length of 5 × 32 bits = 160 bits = 20 bytes. As a 4-bit field, the
           maximum value is 15, this means that the maximum size of the IPv4
           header is 15 × 32 bits = 480 bits = 60 bytes.

   Differentiated Services Code Point (
   Link: mw-deduplicated-inline-style
   DSCP)
           Originally defined as the type of service (ToS), this field
           specifies differentiated services (DiffServ) per RFC 2474.^[a]
           Real-time data streaming makes use of the DSCP field. An example
           is Voice over IP (VoIP), which is used for interactive voice
           services.

   Explicit Congestion Notification (
   Link: mw-deduplicated-inline-style
   ECN)
           This field is defined in RFC 3168 and allows end-to-end
           notification of network congestion without dropping packets. ECN
           is an optional feature available when both endpoints support it
           and effective when also supported by the underlying network.

   Link: mw-deduplicated-inline-style
   Total Length
           This 16-bit field defines the entire packet size in bytes,
           including header and data. The minimum size is 20 bytes (header
           without data) and the maximum is 65,535 bytes. All hosts are
           required to be able to reassemble datagrams of size up to 576
           bytes, but most modern hosts handle much larger packets. Links may
           impose further restrictions on the packet size, in which case
           datagrams must be fragmented. Fragmentation in IPv4 is performed
           in either the sending host or in routers. Reassembly is performed
           at the receiving host.

   Link: mw-deduplicated-inline-style
   Identification
           This field is an identification field and is primarily used for
           uniquely identifying the group of fragments of a single IP
           datagram. Some experimental work has suggested using the ID field
           for other purposes, such as for adding packet-tracing information
           to help trace datagrams with spoofed source addresses,^[38] but
           RFC 6864 now prohibits any such use.

   Link: mw-deduplicated-inline-style
   Flags
           A three-bit field follows and is used to control or identify
           fragments. They are (in order, from most significant to least
           significant):
              * bit 0: Reserved; must be zero.^[b]
              * bit 1: Don't Fragment (DF)
              * bit 2: More Fragments (MF)
           If the DF flag is set, and fragmentation is required to route the
           packet, then the packet is dropped. This can be used when sending
           packets to a host that does not have resources to perform
           reassembly of fragments. It can also be used for path MTU
           discovery, either automatically by the host IP software, or
           manually using diagnostic tools such as ping or traceroute.
           For unfragmented packets, the MF flag is cleared. For fragmented
           packets, all fragments except the last have the MF flag set. The
           last fragment has a non-zero Fragment Offset field,
           differentiating it from an unfragmented packet.

   Link: mw-deduplicated-inline-style
   Fragment offset
           This field specifies the offset of a particular fragment relative
           to the beginning of the original unfragmented IP datagram in units
           of eight-byte blocks. The first fragment has an offset of zero.
           The 13 bit field allows a maximum offset of (2^13 – 1) × 8 =
           65,528 bytes, which, with the header length included (65,528 + 20
           = 65,548 bytes), supports fragmentation of packets exceeding the
           maximum IP length of 65,535 bytes.

   Link: mw-deduplicated-inline-style
   Time to live (TTL)
           An eight-bit time to live field limits a datagram's lifetime to
           prevent network failure in the event of a routing loop. It is
           specified in seconds, but time intervals less than 1 second are
           rounded up to 1. In practice, the field is used as a hop
           count—when the datagram arrives at a router, the router decrements
           the TTL field by one. When the TTL field hits zero, the router
           discards the packet and typically sends an ICMP time exceeded
           message to the sender.
           The program traceroute sends messages with adjusted TTL values and
           uses these ICMP time exceeded messages to identify the routers
           traversed by packets from the source to the destination.

   Link: mw-deduplicated-inline-style
   Protocol
           This field defines the protocol used in the data portion of the IP
           datagram. IANA maintains a list of IP protocol numbers as directed
           by RFC 790.

   Link: mw-deduplicated-inline-style
   Header checksum
           The 16-bit IPv4 header checksum field is used for error-checking
           of the header. When a packet arrives at a router, the router
           calculates the checksum of the header and compares it to the
           checksum field. If the values do not match, the router discards
           the packet. Errors in the data field must be handled by the
           encapsulated protocol. Both UDP and TCP have separate checksums
           that apply to their data.
           When a packet arrives at a router, the router decreases the TTL
           field in the header. Consequently, the router must calculate a new
           header checksum.

   Link: mw-deduplicated-inline-style
   Source address
           This field is the IPv4 address of the sender of the packet. Note
           that this address may be changed in transit by a network address
           translation device.

   Link: mw-deduplicated-inline-style
   Destination address
           This field is the IPv4 address of the receiver of the packet. As
           with the source address, this may be changed in transit by a
           network address translation device.

   Link: mw-deduplicated-inline-style
   Options
           The options field is not often used. Packets containing some
           options may be considered as dangerous by some routers and be
           blocked.^[39] Note that the value in the IHL field must include
           enough extra 32-bit words to hold all the options plus any padding
           needed to ensure that the header contains an integer number of
           32-bit words. If IHL is greater than 5 (i.e., it is from 6 to 15)
           it means that the options field is present and must be considered.
           The list of options may be terminated with an EOOL (End of Options
           List, 0x00) option; this is only necessary if the end of the
           options would not otherwise coincide with the end of the header.
           The possible options that can be put in the header are as follows:

           Field         Size (bits) Description                              
                                     Set to 1 if the options need to be       
           Copied        1           copied into all fragments of a           
                                     fragmented packet.                       
                                     A general options category. 0 is for     
           Option Class  2           control options, and 2 is for debugging  
                                     and measurement. 1 and 3 are reserved.   
           Option Number 5           Specifies an option.                     
                                     Indicates the size of the entire option  
           Option Length 8           (including this field). This field may   
                                     not exist for simple options.            
           Option Data   Variable    Option-specific data. This field may not 
                                     exist for simple options.                

           The table below shows the defined options for IPv4. The Option
           Type column is derived from the Copied, Option Class, and Option
           Number bits as defined above.^[40]

           Option Type (decimal /       Option Name Description               
           hexadecimal)                 
           0 / 0x00                     EOOL        End of Option List        
           1 / 0x01                     NOP         No Operation              
           2 / 0x02                     SEC         Security (defunct)        
           7 / 0x07                     RR          Record Route              
           10 / 0x0A                    ZSU         Experimental Measurement  
           11 / 0x0B                    MTUP        MTU Probe                 
           12 / 0x0C                    MTUR        MTU Reply                 
           15 / 0x0F                    ENCODE      ENCODE                    
           25 / 0x19                    QS          Quick-Start               
           30 / 0x1E                    EXP         RFC3692-style Experiment  
           68 / 0x44                    TS          Time Stamp                
           82 / 0x52                    TR          Traceroute                
           94 / 0x5E                    EXP         RFC3692-style Experiment  
           130 / 0x82                   SEC         Security (RIPSO)          
           131 / 0x83                   LSR         Loose Source Route        
           133 / 0x85                   E-SEC       Extended Security (RIPSO) 
           134 / 0x86                   CIPSO       Commercial IP Security    
                                                    Option                    
           136 / 0x88                   SID         Stream ID                 
           137 / 0x89                   SSR         Strict Source Route       
           142 / 0x8E                   VISA        Experimental Access       
                                                    Control                   
           144 / 0x90                   IMITD       IMI Traffic Descriptor    
           145 / 0x91                   EIP         Extended Internet         
                                                    Protocol                  
           147 / 0x93                   ADDEXT      Address Extension         
           148 / 0x94                   RTRALT      Router Alert              
           149 / 0x95                   SDB         Selective Directed        
                                                    Broadcast                 
           151 / 0x97                   DPS         Dynamic Packet State      
           152 / 0x98                   UMP         Upstream Multicast Pkt.   
           158 / 0x9E                   EXP         RFC3692-style Experiment  
           205 / 0xCD                   FINN        Experimental Flow Control 
           222 / 0xDE                   EXP         RFC3692-style Experiment  

  DataEdit

   The packet payload is not included in the checksum. Its contents are
   interpreted based on the value of the Protocol header field.

   List of IP protocol numbers contains a complete list of payload protocol
   types. Some of the common payload protocols include:

   Protocol Number Protocol Name                        Abbreviation 
   1               Internet Control Message Protocol    ICMP         
   2               Internet Group Management Protocol   IGMP         
   6               Transmission Control Protocol        TCP          
   17              User Datagram Protocol               UDP          
   41              IPv6 encapsulation                   ENCAP        
   89              Open Shortest Path First             OSPF         
   132             Stream Control Transmission Protocol SCTP         

Fragmentation and reassemblyEdit

   Link: mw-deduplicated-inline-style
   Main article: IP fragmentation

   The Internet Protocol enables traffic between networks. The design
   accommodates networks of diverse physical nature; it is independent of the
   underlying transmission technology used in the link layer. Networks with
   different hardware usually vary not only in transmission speed, but also
   in the maximum transmission unit (MTU). When one network wants to transmit
   datagrams to a network with a smaller MTU, it may fragment its datagrams.
   In IPv4, this function was placed at the Internet Layer and is performed
   in IPv4 routers limiting exposure to these issues by hosts.

   In contrast, IPv6, the next generation of the Internet Protocol, does not
   allow routers to perform fragmentation; hosts must perform Path MTU
   Discovery before sending datagrams.

  FragmentationEdit

   When a router receives a packet, it examines the destination address and
   determines the outgoing interface to use and that interface's MTU. If the
   packet size is bigger than the MTU, and the Do not Fragment (DF) bit in
   the packet's header is set to 0, then the router may fragment the packet.

   The router divides the packet into fragments. The maximum size of each
   fragment is the outgoing MTU minus the IP header size (20 bytes minimum;
   60 bytes maximum). The router puts each fragment into its own packet, each
   fragment packet having the following changes:

     * The total length field is the fragment size.
     * The more fragments (MF) flag is set for all fragments except the last
       one, which is set to 0.
     * The fragment offset field is set, based on the offset of the fragment
       in the original data payload. This is measured in units of 8-byte
       blocks.
     * The header checksum field is recomputed.

   For example, for an MTU of 1,500 bytes and a header size of 20 bytes, the
   fragment offsets would be multiples of {\displaystyle {\frac
   {1{,}500-20}{8}}=185}  (0, 185, 370, 555, 740, etc.).

   It is possible that a packet is fragmented at one router, and that the
   fragments are further fragmented at another router. For example, a packet
   of 4,520 bytes, including a 20 bytes IP header is fragmented to two
   packets on a link with an MTU of 2,500 bytes:

   Fragment Size    Header size Data size Flag           Fragment offset 
            (bytes) (bytes)     (bytes)   More fragments (8-byte blocks) 
   1        2,500   20          2,480     1              0               
   2        2,040   20          2,020     0              310             

   The total data size is preserved: 2,480 bytes + 2,020 bytes = 4,500 bytes.
   The offsets are {\displaystyle 0}  and {\displaystyle {\frac
   {0+2{,}480}{8}}=310} .

   When forwarded to a link with an MTU of 1,500 bytes, each fragment is
   fragmented into two fragments:

   Fragment Size    Header size Data size Flag           Fragment offset 
            (bytes) (bytes)     (bytes)   More fragments (8-byte blocks) 
   1        1,500   20          1,480     1              0               
   2        1,020   20          1,000     1              185             
   3        1,500   20          1,480     1              310             
   4        560     20          540       0              495             

   Again, the data size is preserved: 1,480 + 1,000 = 2,480, and 1,480 + 540
   = 2,020.

   Also in this case, the More Fragments bit remains 1 for all the fragments
   that came with 1 in them and for the last fragment that arrives, it works
   as usual, that is the MF bit is set to 0 only in the last one. And of
   course, the Identification field continues to have the same value in all
   re-fragmented fragments. This way, even if fragments are re-fragmented,
   the receiver knows they have initially all started from the same packet.

   The last offset and last data size are used to calculate the total data
   size: {\displaystyle 495\times 8+540=3{,}960+540=4{,}500} .

  ReassemblyEdit

   A receiver knows that a packet is a fragment, if at least one of the
   following conditions is true:

     * The flag more fragments is set, which is true for all fragments except
       the last.
     * The field fragment offset is nonzero, which is true for all fragments
       except the first.

   The receiver identifies matching fragments using the source and
   destination addresses, the protocol ID, and the identification field. The
   receiver reassembles the data from fragments with the same ID using both
   the fragment offset and the more fragments flag. When the receiver
   receives the last fragment, which has the more fragments flag set to 0, it
   can calculate the size of the original data payload, by multiplying the
   last fragment's offset by eight and adding the last fragment's data size.
   In the given example, this calculation was {\displaystyle 495\times
   8+540=4{,}500}  bytes. When the receiver has all fragments, they can be
   reassembled in the correct sequence according to the offsets to form the
   original datagram.

Assistive protocolsEdit

   IP addresses are not tied in any permanent manner to networking hardware
   and, indeed, in modern operating systems, a network interface can have
   multiple IP addresses. In order to properly deliver an IP packet to the
   destination host on a link, hosts and routers need additional mechanisms
   to make an association between the hardware address^[c] of network
   interfaces and IP addresses. The Address Resolution Protocol (ARP)
   performs this IP-address-to-hardware-address translation for IPv4. In
   addition, the reverse correlation is often necessary. For example, unless
   an address is preconfigured by an administrator, when an IP host is booted
   or connected to a network it needs to determine its IP address. Protocols
   for such reverse correlations include Dynamic Host Configuration Protocol
   (DHCP), Bootstrap Protocol (BOOTP) and, infrequently, reverse ARP.

See alsoEdit

     * History of the Internet
     * List of assigned /8 IPv4 address blocks
     * List of IP protocol numbers

NotesEdit

    1. ^ Updated by
       Link: mw-deduplicated-inline-style
       RFC 3168 and
       Link: mw-deduplicated-inline-style
       RFC 3260
    2. ^ As an April Fools' joke, proposed for use in RFC 3514 as the "Evil
       bit"
    3. ^ For IEEE 802 networking technologies, including Ethernet, the
       hardware address is a MAC address.

ReferencesEdit

   Link: mw-deduplicated-inline-style
    1. ^
       Link: mw-deduplicated-inline-style
       "BGP Analysis Reports". Retrieved 2013-01-09.
    2. ^
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       "IPv6 – Google". www.google.com. Retrieved 2022-01-28.
    3. ^
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       "IANA IPv4 Special-Purpose Address Registry". www.iana.org. Retrieved
       2022-01-28.
    4. ^
       Link: mw-deduplicated-inline-style
       "RFC 5735 - Special Use IPv4 Addresses". datatracker.ietf.org.
       Retrieved 2022-01-28.
    5. ^
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       "A Brief History of IPv4". IPv4 Market Group. Retrieved 2020-08-19.
    6. ^
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       "Understanding IP Addressing: Everything You Ever Wanted To Know"
       (PDF). 3Com. Archived from the original (PDF) on June 16, 2001.
    7. ^
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       Cotton, M.; Vegoda, L. (January 2010). Special Use IPv4 Addresses.
       doi:10.17487/RFC5735. RFC 5735.
    8. ^ ^a ^b ^c ^d
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       M. Cotton; L. Vegoda; R. Bonica; B. Haberman (April 2013).
       Special-Purpose IP Address Registries. Internet Engineering Task
       Force. doi:10.17487/RFC6890. BCP 153. RFC 6890. Updated by
       Link: mw-deduplicated-inline-style
       RFC 8190.
    9. ^ ^a ^b ^c ^d
       Link: mw-deduplicated-inline-style
       Y. Rekhter; B. Moskowitz; D. Karrenberg; G. J. de Groot; E. Lear
       (February 1996). Address Allocation for Private Internets. Network
       Working Group. doi:10.17487/RFC1918. BCP 5. RFC 1918. Updated by
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       RFC 6761.
   10. ^
       Link: mw-deduplicated-inline-style
       J. Weil; V. Kuarsingh; C. Donley; C. Liljenstolpe; M. Azinger (April
       2012). IANA-Reserved IPv4 Prefix for Shared Address Space. Internet
       Engineering Task Force (IETF). doi:10.17487/RFC6598. ISSN 2070-1721.
       BCP 153. RFC 6598.
   11. ^
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       S. Cheshire; B. Aboba; E. Guttman (May 2005). Dynamic Configuration of
       IPv4 Link-Local Addresses. Network Working Group.
       doi:10.17487/RFC3927. RFC 3927.
   12. ^ ^a ^b ^c
       Link: mw-deduplicated-inline-style
       J. Arkko; M. Cotton; L. Vegoda (January 2010). IPv4 Address Blocks
       Reserved for Documentation. Internet Engineering Task Force.
       doi:10.17487/RFC5737. ISSN 2070-1721. RFC 5737.
   13. ^
       Link: mw-deduplicated-inline-style
       O. Troan (May 2015). B. Carpenter (ed.). Deprecating the Anycast
       Prefix for 6to4 Relay Routers. Internet Engineering Task Force.
       doi:10.17487/RFC7526. BCP 196. RFC 7526.
   14. ^
       Link: mw-deduplicated-inline-style
       C. Huitema (June 2001). An Anycast Prefix for 6to4 Relay Routers.
       Network Working Group. doi:10.17487/RFC3068. RFC 3068. Obsoleted by
       Link: mw-deduplicated-inline-style
       RFC 7526.
   15. ^
       Link: mw-deduplicated-inline-style
       S. Bradner; J. McQuaid (March 1999). Benchmarking Methodology for
       Network Interconnect Devices. Network Working Group.
       doi:10.17487/RFC2544. RFC 2544. Updated by:
       Link: mw-deduplicated-inline-style
       RFC 6201 and
       Link: mw-deduplicated-inline-style
       RFC 6815.
   16. ^ ^a ^b
       Link: mw-deduplicated-inline-style
       M. Cotton; L. Vegoda; D. Meyer (March 2010). IANA Guidelines for IPv4
       Multicast Address Assignments. Internet Engineering Task Force.
       doi:10.17487/RFC5771. BCP 51. RFC 5771.
   17. ^
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       S. Venaas; R. Parekh; G. Van de Velde; T. Chown; M. Eubanks (August
       2012). Multicast Addresses for Documentation. Internet Engineering
       Task Force. doi:10.17487/RFC6676. RFC 6676.
   18. ^
       Link: mw-deduplicated-inline-style
       J. Reynolds, ed. (January 2002). Assigned Numbers: RFC 1700 is
       Replaced by an On-line Database. Network Working Group.
       doi:10.17487/RFC3232. RFC 3232. Obsoletes
       Link: mw-deduplicated-inline-style
       RFC 1700.
   19. ^
       Link: mw-deduplicated-inline-style
       Jeffrey Mogul (October 1984). Broadcasting Internet Datagrams. Network
       Working Group. doi:10.17487/RFC0919. RFC 919.
   20. ^
       Link: mw-deduplicated-inline-style
       "RFC 923". IETF. June 1984. Retrieved 15 November 2019. Special
       Addresses: In certain contexts, it is useful to have fixed addresses
       with functional significance rather than as identifiers of specific
       hosts. When such usage is called for, the address zero is to be
       interpreted as meaning "this", as in "this network".
   21. ^
       Link: mw-deduplicated-inline-style
       Robert Braden (October 1989). "Requirements for Internet Hosts –
       Communication Layers". IETF. p. 31. RFC 1122.
   22. ^
       Link: mw-deduplicated-inline-style
       Robert Braden (October 1989). "Requirements for Internet Hosts –
       Communication Layers". IETF. p. 66. RFC 1122.
   23. ^
       Link: mw-deduplicated-inline-style
       RFC 3021
   24. ^
       Link: mw-deduplicated-inline-style
       Almquist, Philip; Kastenholz, Frank (November 1994). "Towards
       Requirements for IP Routers". {{cite journal}}: Cite journal requires
       |journal= (help)
   25. ^
       Link: mw-deduplicated-inline-style
       RFC 1916
   26. ^
       Link: mw-deduplicated-inline-style
       RFC 1716
   27. ^
       Link: mw-deduplicated-inline-style
       RFC 1812
   28. ^
       Link: mw-deduplicated-inline-style
       "Understanding and Configuring the ip unnumbered Command". Cisco.
       Retrieved 2021-11-25.
   29. ^
       Link: mw-deduplicated-inline-style
       "World 'running out of Internet addresses'". Archived from the
       original on 2011-01-25. Retrieved 2011-01-23.
   30. ^
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       Smith, Lucie; Lipner, Ian (3 February 2011). "Free Pool of IPv4
       Address Space Depleted". Number Resource Organization. Retrieved 3
       February 2011.
   31. ^
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       ICANN,nanog mailing list. "Five /8s allocated to RIRs – no unallocated
       IPv4 unicast /8s remain".
   32. ^
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       Asia-Pacific Network Information Centre (15 April 2011). "APNIC IPv4
       Address Pool Reaches Final /8". Archived from the original on 7 August
       2011. Retrieved 15 April 2011.
   33. ^
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       Hinden, Bob; Deering, Steve E. (December 1998). "Internet Protocol,
       Version 6 (IPv6) Specification". tools.ietf.org. Retrieved 2019-12-13.
   34. ^
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       Fink, R.; HInden, R. (March 2004). 6bone (IPv6 Testing Address
       Allocation) Phaseout. doi:10.17487/RFC3701. RFC 3701.
   35. ^
       Link: mw-deduplicated-inline-style
       2016 IEEE International Conference on Emerging Technologies and
       Innovative Business Practices for the Transformation of Societies
       (EmergiTech) : date, 3-6 Aug. 2016. University of Technology,
       Mauritius, Institute of Electrical and Electronics Engineers.
       Piscataway, NJ. 2016. ISBN 9781509007066. OCLC 972636788.{{cite
       book}}: CS1 maint: others (link)
   36. ^
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       Partridge, C.; Kastenholz, F. (December 1994). "6.2 IP Header
       Checksum". Technical Criteria for Choosing IP The Next Generation
       (IPng). p. 26. sec. 6.2. doi:10.17487/RFC1726. RFC 1726.
   37. ^
       Link: mw-deduplicated-inline-style
       Postel, J. Internet Protocol. doi:10.17487/RFC0791. RFC 791.
   38. ^
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       Savage, Stefan (2000). "Practical network support for IP traceback".
       ACM SIGCOMM Computer Communication Review. 30 (4): 295–306.
       doi:10.1145/347057.347560. Retrieved 2010-09-06.
   39. ^
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       "Cisco unofficial FAQ". Retrieved 2012-05-10.
   40. ^
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       "Internet Protocol Version 4 (IPv4) Parameters".

External linksEdit

   IPv4at Wikipedia's sister projects
     *  Definitions from Wiktionary
     *  Media from Commons
     *  Resources from Wikiversity
     * Internet Assigned Numbers Authority (IANA)
     * IP, Internet Protocol — IP Header Breakdown, including specific
       options
     * Link: mw-deduplicated-inline-style
       RFC 3344 — IPv4 Mobility
     * IPv6 vs. carrier-grade NAT/squeezing more out of IPv4
     * RIPE report on address consumption as of October 2003
     * Official current state of IPv4 /8 allocations, as maintained by IANA
     * Dynamically generated graphs of IPv4 address consumption with
       predictions of exhaustion dates—Geoff Huston
     * IP addressing in China and the myth of address shortage
     * Countdown of remaining IPv4 available addresses (estimated)
   Retrieved from
   "https://en.wikipedia.org/w/index.php?title=IPv4&oldid=1081194704"
   Last edited on 5 April 2022, at 21:53
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