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Networking Fundamentals · Intermediate

Fragmentation

Splitting an IP packet into smaller pieces so it can cross a link with a smaller MTU, then reassembling it at the destination.

Definition

IPv4 routers can fragment packets in transit unless the Don't Fragment bit is set, using the Identification, Flags and Fragment Offset header fields. IPv6 routers never fragment; only the source host can, using a Fragment extension header, after learning the path MTU. Fragmentation adds overhead and can be abused in evasion and denial-of-service attacks.

Example

A 4000-byte IPv4 packet crossing a 1500-byte MTU link is split into three fragments.

Exam tip

In IPv6 only the sending host fragments; routers send ICMPv6 Packet Too Big instead.

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