IPv4

IPv4 is the version of the Internet Protocol defined in RFC 791 in 1981, and it still carries much of the internet’s traffic. Each address is 32 bits long, conventionally shown as four decimal values from 0 to 255 separated by dots, for example 192.0.2.10. Thirty-two bits allow about 4.3 billion addresses, far fewer than the number of connected devices, and the unallocated pool held by the internet registries is now effectively exhausted.

Several techniques extend its life. Classless Inter-Domain Routing replaced the original fixed address classes with flexible prefixes, and subnetting divides a block into smaller networks. Private IP addresses, reserved in RFC 1918, can be reused inside any organisation, with network address translation mapping them to shared public addresses at the edge. On a local segment, ARP resolves an IPv4 address to a hardware address. Security was not part of the original design: the header carries no authentication or encryption, and IPsec is an optional addition.

Exam relevance: questions in this area tend to turn on the address size and notation, on how IPv4 differs from IPv6, and on the point that address translation conserves addresses rather than acting as a security control, even though it hides internal addressing from outside.