the maximum transmission unit
The maximum transmission unit (MTU) is the largest packet, in bytes, that a particular link can carry in a single frame — the size limit set by the road, not by the cargo. Just as a tunnel has a maximum vehicle height, every link technology caps how big a chunk it will move at once. Ethernet's classic MTU is 1500 bytes; other links (Wi-Fi, DSL, tunnels, VPNs) have their own, sometimes smaller, limits.
The MTU matters because an IP datagram must fit inside the link-layer frame of whatever link it is crossing. If a datagram is bigger than the next link's MTU, something has to give: the datagram is either fragmented into smaller pieces (in IPv4) or, if it is marked do-not-fragment, dropped with an ICMP error telling the sender to use a smaller size. The smallest MTU along the whole path, called the path MTU, is the real ceiling — a 1500-byte packet that meets a 1400-byte link mid-path cannot squeeze through unchanged.
Because fragmentation is wasteful and IPv6 forbids routers from doing it, hosts try to discover the path MTU in advance (Path MTU Discovery): they send progressively and watch for ICMP 'packet too big' replies. A practical gotcha is that overly strict firewalls block those ICMP messages, breaking discovery and causing mysterious hangs where small packets work but large ones silently vanish — the classic 'MTU black hole'.
A 4000-byte IP datagram heading onto an Ethernet link (MTU 1500) cannot pass as-is. In IPv4 the router splits it into three fragments of roughly 1480, 1480, and 1040 payload bytes; the destination reassembles them back into the original 4000-byte datagram.
The link's MTU caps frame size; bigger datagrams must fragment or be dropped.
MTU is a property of a link, not of a host or a connection. The bottleneck is the smallest MTU on the path; firewalls that drop the ICMP needed for Path MTU Discovery cause hard-to-diagnose stalls.