IP fragmentation is the process of breaking an IP packet into smaller fragments when it is larger than the maximum transmission unit (MTU) of the next link. The destination, not the intermediate routers, reassembles the fragments into the original packet.
Why it matters
Fragmentation costs performance and reliability. If any single fragment is lost, the whole packet is useless and must be resent. Firewalls may drop fragments because they cannot inspect them without the first piece, and attackers have abused overlapping fragments to evade filters. In IPv4 routers can fragment on the way; in IPv6 only the sending host may fragment, so it relies on path MTU discovery.
Path MTU discovery sets the Don't Fragment flag and uses ICMP "fragmentation needed" messages to learn the smallest MTU on the path.
Example
A 4000-byte packet reaches a link with an MTU of 1500. It is split into three fragments: two carrying 1480 bytes of data and one carrying the remaining 1040 bytes, each with a fragment offset so the receiver can order them. Tunnels and VPNs, which add headers, often trigger this.
Common mistakes
- Blocking all ICMP, which breaks path MTU discovery and causes connections that hang on large transfers.
- Setting an MTU that does not account for VPN or PPPoE overhead.
- Assuming fragmentation is normal; modern TCP usually avoids it through MSS negotiation.
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