IP versions

IP versions

Two versions of the Internet Protocol (IP) are currently in use (see IP version history for details), IP Version 4 and IP Version 6. Each version defines an IP address differently. Because of its prevalence, the generic term IP address typically still refers to the addresses defined by IPv4.

An illustration of an IP address (version 4), in both dot-decimal notation and binary.

[edit] IP version 4 addresses

Main article: IPv4#Addressing

IPv4 uses 32-bit (4-byte) addresses, which limits the address space to 4,294,967,296 (2³²) possible unique addresses. IPv4 reserves some addresses for special purposes such as private networks (~18 million addresses) or multicast addresses (~270 million addresses). This reduces the number of addresses that can be allocated to end users and, as the number of addresses available is consumed, IPv4 address exhaustion is inevitable. This foreseeable shortage was the primary motivation for developing IPv6, which is in various deployment stages around the world and is the only strategy for IPv4 replacement and continued Internet expansion.

IPv4 addresses are usually represented in dot-decimal notation (four numbers, each ranging from 0 to 255, separated by dots, e.g. 208.77.188.166). Each part represents 8 bits of the address, and is therefore called an octet. In less common cases of technical writing, IPv4 addresses may be presented in hexadecimal, octal, or binary representations. In most representations each octet is converted individually.

[edit] IPv4 networks

In the early stages of development of the Internet protocol,^[1] network administrators interpreted an IP address as a structure of network number and host number. The highest order octet (most significant eight bits) was designated the network number and the rest of the bits were called the rest field or host identifier and were used for host numbering within a network. This method soon proved inadequate as additional networks developed that were independent from the existing networks already designated by a network number. In 1981, the Internet addressing specification was revised with the introduction of classful network architecture.^[2]

Classful network design allowed for a larger number of individual network assignments. The first three bits of the most significant octet of an IP address was defined as the class of the address. Three classes (A, B, and C) were defined for universal unicast addressing. Depending on the class derived, the network identification was based on octet boundary segments of the entire address. Each class used successively additional octets in the network identifier, thus reducing the possible number of hosts in the higher order classes (B and C). The following table gives an overview of this system.

Class	First octet in binary	Range of first octet	Network ID	Host ID	Possible number of networks	Possible number of hosts
A	0XXXXXXX	0 - 127	a	b.c.d	27 = 128	224 - 2 = 16,777,214
B	10XXXXXX	128 - 191	a.b	c.d	214 = 16,384	216 - 2 = 65,534
C	110XXXXX	192 - 223	a.b.c	d	221 = 2,097,152	28 - 2 = 254

The articles 'subnetwork' and 'classful network' explain the details of this design.

Although classful network design was a successful developmental stage, it proved unscalable in the rapid expansion of the Internet and was abandoned when Classless Inter-Domain Routing (CIDR) was created for the allocation of IP address blocks and new rules of routing protocol packets using IPv4 addresses. CIDR is based on variable-length subnet masking (VLSM) to allow allocation and routing on arbitrary-length prefixes.

Today, remnants of classful network concepts function only in a limited scope as the default configuration parameters of some network software and hardware components (e.g. netmask), and in the technical jargon used in network administrators' discussions.