IPv4
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, even with the ongoing deployment of Internet Protocol version 6 (IPv6), its successor. IPv4 uses a 32-bit address space which provides 4,294,967,296 (2) unique addresses, but large blocks are reserved for special networking purposes. History . 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. Purpose. 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). Addressing. IPv4 uses 32-bit addresses which limits the address space to (2) addresses. IPv4 reserves special address blocks for private networks (~18 million addresses) and multicast addresses (~270 million addresses). Address representations. 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". Allocation. 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 . This division was made more flexible with the introduction of variable-length subnet masks (VLSM) in in 1987. In 1993, based on this work, introduced Classless Inter-Domain Routing (CIDR), 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 addresses. The Internet Engineering Task Force (IETF) and IANA have restricted from general use various reserved IP addresses for special purposes. Notably these addresses are used  for multicast traffic and to provide addressing space for unrestricted uses on private networks. Private networks. 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. 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 addressing. 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". Loopback . 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 addresses. 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. 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. 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. 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. 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. 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. Address resolution. 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 interface. 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. 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 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. 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: 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. 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. The long-term solution to address exhaustion was the 1998 specification of a new version of the Internet Protocol, IPv6. 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  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. Completion of IPv6 deployment is expected to take considerable time, so that intermediate transition technologies are necessary to permit hosts to participate in the Internet using both versions of the protocol. Packet structure. 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. Header. 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. Data. 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: Fragmentation and reassembly. 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. Fragmentation. 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 formula_1 (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: The total data size is preserved: 2,480 bytes + 2,020 bytes = 4,500 bytes. The offsets are formula_2 and formula_3. When forwarded to a link with an MTU of 1,500 bytes, each fragment is fragmented into two fragments: 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: formula_4. Reassembly. 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 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 formula_5 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 protocols. 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 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.