IPheaderen

IPheaderen are the fundamental part of every Internet Protocol (IP) packet, containing the addressing and control information that routers use to forward data from sender to recipient. The header provides routing instructions, supports fragmentation and reassembly, and carries information used by higher layers for delivery and error reporting. Its structure and fields differ between IPv4 and IPv6, but in both cases the header sits at the network layer and is processed by routers along the path.

IPv4 header

The IPv4 header is variable in length, with a minimum of 20 bytes. The first field indicates the version (4) and the Internet Header Length (IHL). The Differentiated Services Code Point and Explicit Congestion Notification occupy the DSCP/ECN field. Other fields include total length, identification, flags, fragment offset, time-to-live (TTL), protocol, and a header checksum. The header also contains the source and destination IPv4 addresses. Optional and variable-length fields may be present in the options area, extending the header length and padding as needed. IPv4 supports fragmentation, where routers may break packets to fit the path MTU; reassembly occurs at the destination.

IPv6 header

IPv6 uses a fixed 40-byte base header. Its fields include version (value 6), traffic class, flow label, payload length, next header, hop limit, and 128-bit source and destination addresses. There is no header checksum in IPv6; error detection is handled by higher-layer protocols and link-layer mechanisms. Options and additional control information are carried in extension headers that follow the base header. Fragmentation in IPv6 is typically performed by the end hosts, using the Fragment extension header and Path MTU Discovery, rather than by intermediate routers.

Impact and usage

IPheaders enable fundamental network functions such as routing, delivery, and compatibility across diverse networks. They underpin diagnostics, traffic analysis, QoS, and security considerations, and their design differences reflect the evolution from IPv4 to IPv6 and the changing needs of Internet traffic.