IPv4

IPv4
Name	Internet Protocol version 4
Developer	DARPA
Introduction	September 1981
Based on	TCP
Osi layer	Internet layer
Rfcs	RFC 791

IPv4. Internet Protocol version 4 is the foundational communication protocol that routes most traffic across the Internet and other packet-switched networks. It operates as a connectionless protocol within the Internet protocol suite, providing a logical addressing system to identify devices. First deployed by the DARPA for the ARPANET in 1983, it is defined by RFC 791 published by the IETF.

Technical specification

The protocol functions at the Internet layer of the TCP/IP model, sitting above link-layer technologies like Ethernet and below transport-layer protocols such as TCP and UDP. Its primary role is the internetworking of heterogeneous networks, enabling end-to-end datagram delivery across interconnected systems. IPv4 uses best-effort delivery, meaning it does not guarantee delivery, proper sequencing, or duplicate protection, relying on upper-layer protocols for these functions. Key operational mechanisms include fragmentation and time-to-live to manage datagram transit across diverse network paths.

Addressing

The system employs 32-bit addresses, conventionally expressed in dotted-decimal notation as four octets separated by periods, such as `192.0.2.1`. This format provides a theoretical maximum of approximately 4.3 billion unique addresses. The address space is hierarchically divided into two main parts: a network prefix and a host identifier, a concept formalized by classful addressing and later by CIDR. The allocation of these addresses is managed globally by the IANA, which delegates blocks to regional Internet registries like the ARIN and the RIPE NCC.

Address space exhaustion

The depletion of unallocated addresses became a critical issue due to the exponential growth of the Internet and connected devices. The IETF recognized this limitation as early as the 1990s, leading to the development of its successor, IPv6. Key factors accelerating exhaustion included the inefficient initial classful allocation system and the proliferation of personal computing and mobile technology. Final milestones were reached when IANA allocated its last `/8` blocks to the RIRs in 2011, and ARIN exhausted its free pool in 2015, prompting widespread adoption of NAT and IPv6 transition technologies.

Subnetting

This practice involves dividing a larger network into smaller, more manageable subnetworks to improve routing efficiency and security. It is intrinsically linked to CIDR notation, which replaced the rigid classful system by allowing variable-length subnet masks. Subnetting enables organizations to conserve address space and implement logical segmentation within their infrastructure. The process uses a subnet mask to delineate the network portion from the host portion of an address, a fundamental task for network administrators at institutions like Cisco and Juniper Networks.

Special-use addresses

Certain blocks are reserved for specific purposes and are not routed on the public Internet. The IANA maintains this registry, as documented in RFC 1918 and RFC 5735. Notable ranges include private networks like `10.0.0.0/8` and `192.168.0.0/16`, used behind NAT devices. The `127.0.0.0/8` block is designated for loopback testing, typically `127.0.0.1`. Other reservations exist for multicast (`224.0.0.0/4`), link-local addressing (`169.254.0.0/16`), and future use, as defined by the IETF.

Packet structure

A datagram consists of a header and a payload. The minimal header is 20 bytes long but may include options extending it to 60 bytes. Key header fields include the Version, IHL, Differentiated Services Code Point, Time To Live, Protocol identifying the next-layer protocol like TCP, and source and destination addresses. The header concludes with a checksum for error detection. The structure is meticulously defined in RFC 791, ensuring interoperability across devices from vendors like Intel and IBM.

Category:Internet protocols Category:Network layer protocols Category:Internet Standards