Datagram Fragmentation

As a datagram moves from network to network it may be necessary for a router to divide the datagram into smaller pieces. This occurs if a gateway connects two networks that use different physical media, such as ethernet and a serial modem line. Each type of network has a maximum transmission unit (MTU) parameter that defines the largest packet it can transfer. If a datagram is larger than the MTU for the network then the IP module breaks the datagram into smaller parts. This process is called fragmentation. The format of a fragmented datagram is the same as any other datagram. The second 32-bit word of the IP header identifies each fragment of a datagram and tells how to reassemble them back into the original datagram. IP reassembles a fragmented datagram before passing the packet up to the Transport Layer.