Uma MTU de 68 bytes é válida no IPv4 de acordo com RFC 791 :
Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets.
O requisito para o tamanho remontado ser suportado é muito maior:
Every internet destination must be able to receive a datagram of 576 octets either in one piece or in fragments to be reassembled.
No IPv6, esses números foram aumentados para 1280 e 1500 bytes, conforme mencionado em RFC 2460 :
IPv6 requires that every link in the internet have an MTU of 1280 octets or greater. On any link that cannot convey a 1280-octet packet in one piece, link-specific fragmentation and reassembly must be provided at a layer below IPv6.
A node must be able to accept a fragmented packet that, after reassembly, is as large as 1500 octets. A node is permitted to accept fragmented packets that reassemble to more than 1500 octets. An upper-layer protocol or application that depends on IPv6 fragmentation to send packets larger than the MTU of a path should not send packets larger than 1500 octets unless it has assurance that the destination is capable of reassembling packets of that larger size.