MTU and fragmentation

In short: The MTU (maximum transmission unit) is the largest packet size a network link can transmit in one piece. On Ethernet it is usually 1500 bytes.

In more detail: If a packet is larger than the MTU of a segment, it must be split (fragmented) or rejected. A too-small MTU wastes capacity; a too-large one causes problems when a segment cannot handle it.

In Depth

• Path MTU discovery: The sender sets the “Don’t Fragment” bit and adapts the size based on feedback.
• Typical faults: VPNs and tunnels add headers, the effective MTU drops. Small pages load, large ones hang: a classic MTU symptom.
• MSS: With TCP the maximum segment size is negotiated at connection setup.
• Jumbo frames: Up to 9000 bytes in data centers to save overhead.
• Testing: ping -M do -s 1472 target (Linux) checks whether 1500-byte packets get through.

See also: NAT (network address translation)