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  • 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 in the ARPANET in 1983. It still routes most Internet traffic today, despite the ongoing deployment of a successor protocol, IPv6. IPv4 is described in IETF publication RFC 791 September 1981, replacing an earlier definition RFC 760, January 1980.

    IPv4 uses a 32-bit address space, which limits the number of unique hosts to 4,294,967,296 232, but large blocks are reserved for special networking methods.


    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.


    IPv4 uses 32-bit addresses which limits the address space to 294967296 232 addresses.

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

    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 represents the 32-bit decimal number 3221226219, which in hexadecimal format is 0xC00002EB. This may also be expressed in dotted hex format as 0xC0.0x00.0x02.0xEB, or with octal byte values as 0300.0000.0002.0353.

    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 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 was commonly written as 127.1, given that it belonged 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

    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 RFC 950. This division was made more flexible with the introduction of variable-length subnet masks VLSM in RFC 1109 in 1987. In 1993, based on this work, RFC 1517 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.

    The Internet Engineering Task Force IETF and the Internet Assigned Numbers Authority 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.

    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 the transmission across public networks to secure the data.

    RFC 3927 defines the special address block 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.

    The class A network classless network 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.

    The first address in a subnet is used to identify the subnet itself. The last address is used as a local broadcast address for all devices on the subnet.

    For example, in the subnet the identifier commonly is used to refer to the entire subnet. To avoid ambiguity in representation, the address ending in the octet 0 is reserved.

    A broadcast address is an address that allows information to be sent to all interfaces in a given subnet, rather than a specific machine. The broadcast address is the last address in the address range of the subnet. For example, the broadcast address for the network is For networks of size /24 or larger, the broadcast address always ends in 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, which is equivalent to the address range–, the broadcast address is One can use the following addresses for hosts, even though they end with 255:,, etc. Also, is the network identifier and must not be assigned to an interface. The addresses,, 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 has the broadcast address

    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.

    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 which allows for subdelegation of name spaces to other DNS servers.

    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 Classless Inter-Domain Routing CIDR methods 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 the impending address exhaustion was the 1998 specification of a new version of the Internet Protocol, IPv6, which increased the address size to 128 bits. 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 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, and typically the end-to-end TCP layer checksum detects most other errors.

    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.

    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, 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, or 480 bits = 60 bytes.

    The fragment offset field is measured in units of eight-byte blocks. It is 13 bits long and specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram. The first fragment has an offset of zero. This allows a maximum offset of 213 – 1 × 8 = 65,528 bytes, which would exceed the maximum IP packet length of 65,535 bytes with the header length included 65,528 + 20 = 65,548 bytes.

    The 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:

    Packets containing some options may be considered as dangerous by some routers and be blocked.

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

    Some of the common payload protocols are:

    See List of IP protocol numbers for a complete list.

    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, which thus require no implementation of any higher layers for the function of routing IP packets.

    In contrast, IPv6, the next generation of the Internet Protocol, does not allow routers to perform fragmentation; hosts must determine the path MTU 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 max size of each fragment is the 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 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 . These multiples are 0, 185, 370, 555, 740, ...

    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 the 20 bytes of the IP header without options is fragmented to two packets on a link with an MTU of 2,500 bytes:

    The total data size is preserved: 2480 bytes + 2020 bytes = 4500 bytes. The offsets are and .

    On a link with an MTU of 1,500 bytes, each fragment results in two fragments:

    Again, the data size is preserved: 1480 + 1000 = 2480, and 1480 + 540 = 2020.

    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: .

    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 foreign and local address, 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 495*8 + 540 = 4500 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 hardware identifications and, indeed, a network interface can have multiple IP addresses in modern operating systems. Hosts and routers need additional mechanisms to identify the relationship between device interfaces and IP addresses, in order to properly deliver an IP packet to the destination host on a link. The Address Resolution Protocol ARP performs this IP-address-to-hardware-address translation for IPv4. A hardware address is also called a MAC address. In addition, the reverse correlation is often necessary. For example, when an IP host is booted or connected to a network it needs to determine its IP address, unless an address is preconfigured by an administrator. Protocols for such inverse correlations exist in the Internet Protocol Suite. Currently used methods are Dynamic Host Configuration Protocol DHCP, Bootstrap Protocol BOOTP and, infrequently, reverse ARP.