Internet Minimum MTU: 576 Bytes

Each router must be able to fragment as needed to handle IP datagrams up to the size of the largest MTU used by networks to which they attach. Routers are also required, as a minimum, to handle an MTU of at least 576 bytes. This value is specified in RFC 791, and was chosen to allow a “reasonable sized” data block of at least 512 bytes, plus room for the standard IP header and options. Since it is the minimum size specified in the IP standard, 576 bytes has become a common default MTU value used for IP datagrams. Even if a host is connected over a local network with an MTU larger than 576, it may choose to use an MTU value of 576 anyway, to ensure that no further fragmentation will be required by intermediate routers.