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It is one of the core protocols of standards-based internetworking methods in the Internet , and was the first version deployed for production in the ARPANET in It still routes most Internet traffic today,  despite the ongoing deployment of a successor protocol, IPv6. IPv4 is a connectionless protocol for use on packet-switched networks.
It 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 bit addresses which limits the address space to 4 2 32 addresses. IPv4 addresses may be represented in any notation expressing a bit integer value. They are most often written in the 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 This may also be expressed in dotted hex format as 0xC0. In the original design of IPv4, an IP address was divided into two parts: The latter was also called the rest field.
This structure permitted a maximum of network identifiers, which was quickly found to be inadequate. To overcome this limit, the most-significant address octet was redefined in 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, which meant that each network class had a different capacity for addressing hosts.
Class D was defined for multicast addressing and Class E was reserved for future applications. Starting around , methods were devised to subdivide IP networks. One method that has proved flexible is the use of the variable-length subnet mask VLSM. CIDR was designed to permit repartitioning of any address space so that smaller or larger blocks of addresses could be allocated to users.
Some are used for maintenance of routing tables , for multicast traffic, operation under failure modes , or to provide addressing space for public, unrestricted uses on private networks. Of the approximately four billion addresses defined in IPv4, three ranges are reserved for use in private networks. Packets addresses in these ranges are not routable in the public Internet, because 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. Additionally, encapsulated packets may be encrypted for the transmission across public networks to secure the data.
RFC defines the special address block These addresses are only valid on links such as a local network segment or point-to-point connection connected to a host. 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. The class A network IP packets whose source addresses belong to this network should never appear outside a host.
The modus operandi of this network expands upon that of a loopback interface:. Networks with subnet masks of at least 24 bits, i. Classful addressing prescribed only three possible subnet masks: For example, in the 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. Generally, the broadcast address is found by obtaining the bit complement of the subnet mask and performing a bitwise OR operation with the network identifier. In other words, the broadcast address is the last address in the address range of the subnet. For example, the broadcast address for the network However, this does not mean that every address ending in 0 or cannot be used as a host address.
One can use the following addresses for hosts, even though they end with In the past, conflict between network addresses and broadcast addresses arose because some software used non-standard broadcast addresses with zeros instead of ones. For example, a CIDR subnet Hosts on the Internet are usually known by names, e.
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. Since the s, it was apparent that the pool of available IPv4 addresses was being depleted at a rate that was not initially anticipated in the original design of the network address system.
The threat of exhaustion motivated the introduction of a number of remedial technologies, such as classful networks , Classless Inter-Domain Routing CIDR methods, network address translation NAT and strict usage-based allocation policies. To provide a long-term solution to the pending address exhaustion, IPv6 was created in the s, which made many more addresses available by increasing the address size to bits. IPv6 has been in commercial deployment since The accepted and standard long term solution is to use IPv6 which increased the address size to bits, providing a vastly increased address space that 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-only hosts cannot directly communicate with IPv6-only hosts so IPv6 alone does not provide an immediate solution to the IPv4 exhaustion problem. Migration to IPv6 is in progress but completion is expected to take considerable time. 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: 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. When a packet arrives at a router, the router decreases the TTL field. Consequently, the router must calculate a new checksum.
Packets containing some options may be considered as dangerous by some routers and be blocked. The data portion of the packet is not included in the packet checksum. Its contents are interpreted based on the value of the Protocol header field. See List of IP protocol numbers for a complete list. The Internet Protocol enables networks to communicate with one another. 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 only require this layer as the highest one implemented in their design. 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:. These multiples are 0, , , , , 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, bytes, including the 20 bytes of the IP header without options is fragmented to two packets on a link with an MTU of 2, bytes:.
The total data size is preserved: 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 length of the original data payload, by multiplying the last fragment's offset by eight, and adding the last fragment's data size.
When the receiver has all fragments, they can be correctly ordered by using the offsets, and reassembled to yield the original data segment. The Internet Protocol is the protocol that defines and enables internetworking at the Internet Layer and thus forms the Internet. It uses a logical addressing system. IP addresses are not tied in any permanent manner to hardware identifications and, indeed, a network interface can have multiple IP addresses.