Fragmentoffset

In the Internet Protocol version 4 (IPv4), the fragment offset is a 13-bit field in the IP header that indicates where the payload of a fragment belongs within the original datagram. The offset is measured in 8-byte units, and the first fragment has an offset of zero. Each subsequent fragment's offset is equal to the number of eight-byte blocks that precede that fragment's payload.

When a router cannot forward a large datagram without exceeding the path MTU, it may fragment the packet. Each fragment carries its own IP header with an adjusted Total Length and a Fragment Offset value, and all fragments share the same Identification field so the destination can reassemble them. The More Fragments flag (MF) indicates whether more fragments follow; MF is set to 1 on all fragments except the last, which has MF set to 0.

Reassembly occurs at the destination. Fragments are collected and reordered by Identification, Source, Destination, and Fragment Offset. The original datagram is reconstructed only when all fragments arrive within a reassembly timeout. If any fragment is lost, the datagram cannot be reassembled and is discarded.

Fragment offset and fragmentation have security and performance implications. Fragmentation can be abused in packet fragmentation attacks or evasion of filters. Routing devices and network policies aim to minimize fragmentation, using techniques such as Path MTU Discovery and the Don’t Fragment (DF) bit. In modern networks, fragmentation is less common, and IPv6 handles fragmentation differently, with sources fragmenting or using routing headers in limited cases.