IP version 4

(From Wikipedia, the free encyclopedia)


In version 4 of the Internet protocol (IPv4), the current standard protocol for the Internet, IP addresses consist of 32 bits, which makes for 4,294,967,296 (over 4 billion) unique host interface addresses in theory. In practice the address space is sparsely populated due to routing issues, so that there is some pressure to extend the address range via IP version 6.

IPv4 addresses are commonly expressed as a dotted quad, four octets (8 bits) separated by periods. The host known as www.wikipedia.org currently has the number 3482223596, written as in base-256: 3482223596 equals 207×2563 + 142×2562 + 131×2561 + 236×2560. (Resolving the name "www.wikipedia.org" to its associated number is handled by Domain Name System servers.)

Historically, IPv4 addresses originally had only two parts. A later change increased that to three parts: the network part, the subnetwork part, and the host part, in that order. However, with the advent of classless inter-domain routing (CIDR), this is no longer true, and the address can have an arbitrary number of levels of hierarchy. (Technically, this was already true any time after the advent of subnets, since a site could elect to have more than one level of subnetting inside a network number.)



The actual assignment of an address is not arbitrary. An organization, typically an Internet service provider, requests an assignment of a netblock from a registry such as the American Registry for Internet Numbers (ARIN). The network number comprises a range of addresses which the organization is free to allocate as they wish. An organization that has exhausted a significant part of its allocated address space can request another netblock.



Some private IP address space has been allocated via RFC 1918. This means the addresses are available for any use by anyone and therefore the same RFC 1918 IP addresses can be reused. However they are not routable on the Internet. They are used extensively due to the shortage of registerable addresses. Network address translation (NAT) is required to connect those networks to the Internet.

While a number of measures have been taken to conserve the limited existing IPv4 address space (such as the use of NAT and Private Addressing), the number of 32-bit IP addresses is not sufficient to accommodate the long-term growth of the Internet. For this reason, there is a general consensus that the Internet 128-bit IPv6 addressing scheme will be adopted over the next 5 to 15 years.