IPv4

IPv4 is the fourth version of the Internet Protocol and the dominant protocol for routing traffic on the Internet. It uses a 32-bit address space, usually written in dotted decimal form as four octets (for example, 192.0.2.1). IPv4 packets have a header that includes the version, header length, total length, time to live, protocol, and source and destination addresses. The protocol supports unicast, multicast, and, with special handling, broadcast delivery. Fragmentation lets large packets be split to fit network MTUs.

Address space and allocation: The 32-bit space provides about 4.29 billion addresses. Classful addressing (A/B/C with default masks) has been superseded by CIDR for flexible allocation. Private address ranges include 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Public addresses are routable on the Internet. To conserve space, NAT (Network Address Translation) and DHCP are widely used. The IANA delegates address blocks to regional registries and, ultimately, to service providers.

Routing and services: IPv4 relies on routing protocols and the broader Internet routing system. Local networks use ARP to map addresses to hardware addresses. DHCP automates address assignment; ICMP handles error messaging and diagnostics. The IPv4 header supports fragmentation via flags and offset, but modern networks often use path MTU discovery to avoid fragmentation.

Status and transition: Address exhaustion prompted widespread NAT adoption and the development of IPv6, which offers a vastly larger address space. IPv4 remains in active use and is defined in RFC 791, with numerous updates over time. Many networks operate dual-stack configurations to maintain compatibility during transition to IPv6.