Fragmentation splits large packets to fit link MTU.
When IPv4 router encounters an oversized packet (DF not set):
Splits payload into pieces
Copies IP header to each
Sets fragment offset and More Fragments flag
Forwards each independently
Fragment fields:
- Identification: shared ID
- Offset: position (-byte units)
- MF flag: set except for last
IPv6 only allows source fragmentation, not router fragmentation.