| Protocol stack | |
 IPv4 packet | |
| Abbreviation | IPv4 |
|---|---|
| Purpose | Internetworking protocol |
| Developer(s) | DARPA |
| Introduction | 1981; 44 years ago (1981) |
| Influenced | IPv6 |
| OSI layer | Network layer |
| RFC(s) | 791 |
| Internet protocol suite |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Link layer |
Internet Protocol version 4 (IPv4) is the first version of theInternet Protocol (IP) as a standalone specification. It is one of the core protocols of standards-basedinternetworking methods in theInternet and otherpacket-switched networks. IPv4 was the first version deployed for production onSATNET in 1982 and on theARPANET in January 1983. It is still used to route mostInternet traffic today,[1] even with the ongoing deployment ofInternet Protocol version 6 (IPv6),[2] its successor.
IPv4 uses a32-bit address space which provides 4,294,967,296 (232) unique addresses, but large blocks are reserved for special networking purposes.[3][4]
Earlier versions of TCP/IP were a combined specification through TCP/IPv3. With IPv4, the Internet Protocol became a separate specification.[5]
Internet Protocol version 4 is described inIETF publication RFC 791 (September 1981), replacing an earlier definition of January 1980 (RFC 760). In March 1982, the US Department of Defense decided on theInternet Protocol Suite (TCP/IP) as the standard for all militarycomputer networking.[6]
The Internet Protocol is the protocol that defines and enablesinternetworking at theinternet layer of the Internet Protocol Suite. In essence it forms the Internet. It uses a logical addressing system and performsrouting, 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 aconnectionless protocol, and operates on abest-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 anupper layer transport protocol, such as theTransmission Control Protocol (TCP).
IPv4 uses 32-bit addresses which limits theaddress space to4294967296 (232) addresses.
IPv4 reserves special address blocks forprivate networks (224 + 220 + 216 ≈ 18 million addresses) andmulticast addresses (228 ≈ 268 million addresses).
IPv4 addresses may be represented in any notation expressing a 32-bit integer value. They are most often written indot-decimal notation, which consists of fouroctets of the address expressed individually indecimal numbers and separated byperiods.
For example, the quad-dotted IP address in the illustration (172.16.254.1) represents the 32-bitdecimal number 2886794753, which inhexadecimal format is 0xAC10FE01.
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 consecutive1 bits in the routing prefix (subnet mask).
Other address representations were in common use whenclassful networking was practiced. For example, theloopback address127.0.0.1 was commonly written as127.1, 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 were specified in the address in dotted notation, the last value was treated as an integer of as many bytes as are required to fill out the address to four octets. Thus, the address127.65530 is equivalent to127.0.255.250.
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 therest 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 createnetwork classes, in a system which later became known asclassful 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 formulticast addressing and Class E was reserved for future applications.
Dividing existing classful networks into subnets began in 1985 with the publication ofRFC 950. This division was made more flexible with the introduction of variable-length subnet masks (VLSM) inRFC 1109 in 1987. In 1993, based on this work,RFC 1517 introducedClassless Inter-Domain Routing (CIDR),[7] which expressed the number of bits (from themost significant) as, for instance,/24, and the class-based scheme was dubbedclassful, 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 theInternet Assigned Numbers Authority (IANA) and theregional Internet registries (RIRs). Each RIR maintains a publicly searchableWHOIS database that provides information about IP address assignments.
TheInternet Engineering Task Force (IETF) and IANA have restricted from general use variousreserved IP addresses for special purposes.[4] Notably these addresses are used formulticast traffic and to provide addressing space for unrestricted uses on private networks.
| Address block | Address range | Number of addresses | Scope | Description |
|---|---|---|---|---|
| 0.0.0.0/8 | 0.0.0.0–0.255.255.255 | 16777216 | Software | Current (local, "this") network[4] |
| 10.0.0.0/8 | 10.0.0.0–10.255.255.255 | 16777216 | Private network | Used for local communications within a private network[8] |
| 100.64.0.0/10 | 100.64.0.0–100.127.255.255 | 4194304 | Private network | Shared address space[9] for communications between a service provider and its subscribers when using acarrier-grade NAT |
| 127.0.0.0/8 | 127.0.0.0–127.255.255.255 | 16777216 | Host | Used forloopback addresses to the local host[4] |
| 169.254.0.0/16 | 169.254.0.0–169.254.255.255 | 65536 | Subnet | Used forlink-local addresses[10] between two hosts on a single link when no IP address is otherwise specified, such as would have normally been retrieved from aDHCP server |
| 172.16.0.0/12 | 172.16.0.0–172.31.255.255 | 1048576 | Private network | Used for local communications within a private network[8] |
| 192.0.0.0/24 | 192.0.0.0–192.0.0.255 | 256 | Private network | IETF Protocol Assignments,DS-Lite (/29)[4] |
| 192.0.2.0/24 | 192.0.2.0–192.0.2.255 | 256 | Documentation | Assigned as TEST-NET-1, documentation and examples[11] |
| 192.88.99.0/24 | 192.88.99.0–192.88.99.255 | 256 | Internet | Reserved.[12] Formerly used forIPv6 to IPv4 relay[13] (includedIPv6 address block2002::/16). |
| 192.168.0.0/16 | 192.168.0.0–192.168.255.255 | 65536 | Private network | Used for local communications within a private network[8] |
| 198.18.0.0/15 | 198.18.0.0–198.19.255.255 | 131072 | Private network | Used for benchmark testing of inter-network communications between two separate subnets[14] |
| 198.51.100.0/24 | 198.51.100.0–198.51.100.255 | 256 | Documentation | Assigned as TEST-NET-2, documentation and examples[11] |
| 203.0.113.0/24 | 203.0.113.0–203.0.113.255 | 256 | Documentation | Assigned as TEST-NET-3, documentation and examples[11] |
| 224.0.0.0/4 | 224.0.0.0–239.255.255.255 | 268435456 | Internet | In use formulticast[15] (former Class D network) |
| 233.252.0.0/24 | 233.252.0.0–233.252.0.255 | 256 | Documentation | Assigned as MCAST-TEST-NET, documentation and examples (This is part of the above multicast space.)[15][16] |
| 240.0.0.0/4 | 240.0.0.0–255.255.255.254 | 268435455 | Internet | Reserved for future use[17] (former Class E network) |
| 255.255.255.255/32 | 255.255.255.255 | 1 | Subnet | Reserved for the "limitedbroadcast" destination address[4] |
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 requirenetwork address translation at a routing gateway for this purpose.
| Name | CIDR block | Address range | Number of addresses | Classful description |
|---|---|---|---|---|
| 24-bit block | 10.0.0.0/8 | 10.0.0.0 – 10.255.255.255 | 16777216 | Single Class A |
| 20-bit block | 172.16.0.0/12 | 172.16.0.0 – 172.31.255.255 | 1048576 | Contiguous range of 16 Class B blocks |
| 16-bit block | 192.168.0.0/16 | 192.168.0.0 – 192.168.255.255 | 65536 | Contiguous range 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 avirtual private network (VPN) or anIP tunnel, whichencapsulates 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.
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 calledAutomatic Private IP Addressing (APIPA), which was deployed on millions of machines and became ade facto standard. Many years later, in May 2005, theIETF defined a formal standard in RFC 3927, entitledDynamic Configuration of IPv4 Link-Local Addresses.
The class A network127.0.0.0 (classless network127.0.0.0/8) is reserved forloopback. 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.
The first address in a subnet is used to identify the subnet itself. In this address all host bits are0. To avoid ambiguity in representation, this address is reserved.[18] The last address has all host bits set to1. It is used as a localbroadcast 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 subnet192.168.5.0/24 (subnet mask255.255.255.0) the identifier192.168.5.0 is used to refer to the entire subnet. The broadcast address of the network is192.168.5.255.
| Type | Binary form | Dot-decimal notation |
|---|---|---|
| Network space | 11000000.10101000.00000101.00000000 | 192.168.5.0 |
| Broadcast address | 11000000.10101000.00000101.11111111 | 192.168.5.255 |
| 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 subnet192.168.0.0/255.255.0.0, which is equivalent to the address range192.168.0.0–192.168.255.255, the broadcast address is192.168.255.255. 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.[19]: 31 The addresses192.168.1.0,192.168.2.0, 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.[19]: 66
In networks smaller than/24, broadcast addresses do not necessarily end with 255. For example, a CIDR subnet203.0.113.16/28 has the broadcast address203.0.113.31.
| Type | Binary form | Dot-decimal notation |
|---|---|---|
| Network space | 11001011.00000000.01110001.00010000 | 203.0.113.16 |
| Broadcast address | 11001011.00000000.01110001.00011111 | 203.0.113.31 |
| 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.[20]
Hosts on theInternet 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, calledresolving, 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 theDomain Name System (DNS), a hierarchical, distributed naming system that allows for the subdelegation ofnamespaces to other DNS servers.
An unnumberedpoint-to-point (PtP) link, also called a transit link, is a link that does not have an IP network or subnet number associated with it, but still has an IP address. First introduced in 1993,[21][22][23][24] Phil Karn from Qualcomm is credited as the original designer.
The purpose of a transit link is toroutedatagrams. They are used to free IP addresses from a scarce IP address space or to reduce the management of assigning IP and configuration of interfaces. Previously, every link needed to dedicate a/31 or/30 subnet using 2 or 4 IP addresses per point-to-point link. When a link is unnumbered, arouter-id is used, a single IP address borrowed from a defined (normally aloopback) interface. The samerouter-id can be used on multiple interfaces.
One of the disadvantages of unnumbered interfaces is that it is harder to do remote testing and management.
In the 1980s, it became 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.[25] The main market forces that accelerated address depletion included the rapidly growing number of Internet users, who increasingly used mobile computing devices, such aslaptop computers,personal digital assistants (PDAs), andsmart 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:
By the mid-1990s, NAT was used pervasively in network access provider systems, along with 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 fiveRIRs.[26][27]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.[28]
The long-term solution to address exhaustion was the 1998 specification of a new version of the Internet Protocol,IPv6.[29] 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 264 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 the6bone experimental network starting in 2004, permanent formal deployment of IPv6 commenced in 2006.[30] Completion ofIPv6 deployment is expected to take considerable time,[31] so that intermediatetransition technologies are necessary to permit hosts to participate in the Internet using both versions of the protocol.
An IPpacket 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 thelink layer encapsulates IP packets in frames with a CRC footer that detects most errors. Manytransport-layer protocols carried by IP also have their own error checking.[32]: §6.2
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 (network byte order), 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.
| Offset | 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 (4) | IHL | DSCP | ECN | Total Length | |||||||||||||||||||||||||||
| 4 | 32 | Identification | Flags | Fragment Offset | |||||||||||||||||||||||||||||
| 8 | 64 | Time to Live | Protocol | Header Checksum | |||||||||||||||||||||||||||||
| 12 | 96 | Source address | |||||||||||||||||||||||||||||||
| 16 | 128 | Destination address | |||||||||||||||||||||||||||||||
| 20 | 160 | (Options) (if IHL > 5) | |||||||||||||||||||||||||||||||
| ⋮ | ⋮ | ||||||||||||||||||||||||||||||||
| 56 | 448 | ||||||||||||||||||||||||||||||||
| 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 |
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 themaximum transmission unit (MTU). When one network wants to transmit datagrams to a network with a smaller MTU, it mayfragment its datagrams. In IPv4, this function was placed at theInternet 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 performPath MTU Discovery before sending datagrams.
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:
For example, for an MTU of 1,500 bytes and a header size of 20 bytes, the fragment offsets would be multiples of (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 (bytes) | Header size (bytes) | Data size (bytes) | Flag More fragments | Fragment offset (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 and.
When forwarded to a link with an MTU of 1,500 bytes, each fragment is fragmented into two fragments:
| Fragment | Size (bytes) | Header size (bytes) | Data size (bytes) | Flag More fragments | Fragment offset (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, theMore 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:.
A receiver knows that a packet is a fragment, if at least one of the following conditions is true:
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 themore 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 bytes. When the receiver has all fragments, they can be reassembled in the correct sequence according to the offsets to form the original datagram.
IP addresses are not tied in any permanent manner to networking hardware and, indeed, in modernoperating 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[b] of network interfaces and IP addresses. TheAddress 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 includeDynamic Host Configuration Protocol (DHCP),Bootstrap Protocol (BOOTP) and, infrequently,reverse ARP.
This article was adapted from the following source under aCC BY 4.0 license (2022) :Michel Bakni; Sandra Hanbo (2022)."A Survey on Internet Protocol version 4 (IPv4)"(PDF).WikiJournal of Science.doi:10.15347/WJS/2022.002.ISSN 2470-6345.OCLC 9708517136.S2CID 254665961.Wikidata Q104661268.
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".
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