This value is called the MTU ( Maximum Transmission Unit).

On the other hand, the maximum datagram size that can be transmitted without fragmentation is not defined by any specification and varies by link type. For IPv6 that minimal value has been bumped up to 1,280 bytes (see RFC2460). The spec defines the minimal requirement - each physical link must be able to transmit datagrams of at least 68 bytes. ATM uses fixed 53 bytes, classical Ethernet can do between bytes. For example Frame Relay can send datagrams between 46 and 4,470 bytes. All physical connections have inherent datagram size limits, depending on the specific medium they use. IPv4 defines the minimum as 576 bytes, but popular operating systems can cope with very big packets, typically up to 65KiB.

The first value - permitted reassembled packet size - is typically not problematic. Of 576 octets either in one piece or in fragments toД®very internet module must be able to forward a datagram of 68 From the RFC791: Every internet destination must be able to receive a datagram The IPv4 specification defines the minimal requirements. Image by Geoff Huston, reproduced with permission This process is called 'forward' IP fragmentation and the smaller datagrams are called IP fragments. When a packet is too big for a physical link, an intermediate router might chop it into multiple smaller datagrams in order to make it fit. What is the maximum permitted datagram size that can be safely pushed through the physical connections between the hosts? What is the maximum packet size that can be handled by operating systems on both ends? The idea was to split the problem into two separate concerns: This is an old question and the IPv4 RFCs answer it pretty clearly. As opposed to the public telephone network, the internet has a Packet Switched design.

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