Internet Protocol (IP) address
An IP address is a unique numerical label assigned to each host on a computer network. It both identifies the host and locates it within the network’s address space, so that packets can be routed to and from it. IP addresses are defined by the Internet Protocol (IP), which operates at Layer 3 of the OSI model. Two address families are in active use, IPv4 and IPv6.
IPv4
An IPv4 address is 32 bits wide, written as four decimal numbers called octets separated by periods, eg. 192.168.1.1.
Each octet ranges from 0 to 255. The 32-bit space gives roughly 4.3 billion unique addresses, a limit that proved far
too small as the internet grew. The resulting exhaustion drove the adoption of IPv6 and the widespread use of
Network Address Translation (NAT) to share one public address among many private hosts.
IPv6
An IPv6 address is 128 bits wide, written as eight groups of one to four hexadecimal digits separated by colons, eg.
2001:0db8:85a3:0000:0000:8a2e:0370:7334. Consecutive groups of zeros can be collapsed with :: once per address, and
leading zeros within a group can be dropped, so the example shortens to 2001:0db8:85a3::8a2e:370:7334. The 128-bit
space provides about 3.4 × 10^38 addresses, which removes the exhaustion pressure that motivated IPv6’s design.
Network and host portions
An address carries two pieces of information: which network the host belongs to, and which host within that network. The
address is split into a network prefix and a host identifier, and the boundary between them is recorded as a prefix
length. A range of addresses is written in CIDR notation, eg. 192.168.1.0/24, where /24 marks the
first 24 bits as the network prefix and the remaining 8 bits as the host identifier.
Assignment and resolution
A host obtains its address either by static configuration or by leasing one dynamically from a DHCP server. Because people rarely address hosts by number, the Domain Name System maps human-readable domain names to IP addresses, letting applications refer to hosts by name.
Both IPv4 and IPv6 reserve ranges for special purposes, including loopback, private, link-local, and multicast addresses. These are listed in the Internet Protocol entry, alongside the protocol-level differences between IPv4 and IPv6.