What Is IPv4?

IPv4, or Internet Protocol version 4, is the fourth revision of the Internet Protocol and the version that carried essentially all Internet traffic from the early 1980s through the 2000s. It defines the format of datagrams, the addressing scheme that identifies network endpoints, and the fragmentation and reassembly procedures that allow large packets to traverse links with smaller maximum transmission units. IPv4 operates at the network layer of the Internet protocol suite, providing an unreliable, connectionless delivery service that transports packets from source to destination without guaranteeing order or receipt.

The protocol was standardized in September 1981 in IETF RFC 791, which remains the normative specification. RFC 791 built on earlier work in ARPANET and incorporated the addressing model developed through the 1970s at DARPA. The design used 32-bit addresses, which provided approximately 4.3 billion unique values, a number that appeared sufficient at the time but proved inadequate as the Internet expanded globally.

Address Space and Subnetting

IPv4 addresses are 32-bit binary values conventionally written in dotted decimal notation, for example 192.0.2.1. The address space is partitioned into a network prefix and a host identifier using a subnet mask or CIDR prefix length. Early allocations used a fixed class structure (Class A, B, and C) that wasted large blocks of addresses in organizations that needed only a few hundred hosts. Classless inter-domain routing (CIDR), introduced in 1993, replaced the class system with variable-length prefixes, allowing finer aggregation and extending the useful life of the address space. Network address translation (NAT), which allows many devices on a private network to share a single public IPv4 address, became nearly universal for home and enterprise networks after the early 2000s, further slowing address exhaustion but complicating end-to-end connectivity.

Packet Format

An IPv4 datagram consists of a header of at least 20 bytes followed by the payload. The header carries the source and destination addresses, a protocol field identifying the transport layer above (TCP, UDP, ICMP, and others), a time-to-live (TTL) counter that routers decrement and discard if it reaches zero, a flags field and fragment offset for managing fragmentation, and a checksum that allows routers to detect header corruption. The options field allows the header to extend up to 60 bytes, though options are rarely used in practice and some routers handle them in software rather than hardware, introducing latency. The IETF TCP/IP tutorial in RFC 1180 explains how the IPv4 header fields interact with routing and transport layer protocols in day-to-day network operation.

Address Exhaustion and Transition

The Internet Assigned Numbers Authority (IANA) allocated the last blocks of IPv4 address space to the five regional internet registries in February 2011, and the Asia-Pacific registry exhausted its pool the same year. Despite this exhaustion, IPv4 remains in widespread use because NAT, address reuse within private networks, and gradual IPv6 adoption have allowed existing IPv4 infrastructure to remain functional. Transition mechanisms including dual-stack operation, in which hosts support both IPv4 and IPv6 simultaneously, and tunneling protocols such as 6to4 and Teredo allow networks to migrate incrementally. The IETF IPv6 specification RFC 8200 defines the successor protocol that replaces the 32-bit address space with 128-bit addresses and removes the header checksum and fragmentation-by-routers features of IPv4.

Applications

IPv4 continues to serve as the foundation for many networking contexts, including:

• Legacy enterprise and carrier network infrastructure
• Home and small-office broadband networking via NAT
• Transition-period dual-stack deployments alongside IPv6
• Industrial control and embedded systems with long replacement cycles
• Private cloud and data center networks using RFC 1918 address space