How is IP carried on a network?

IP really isn't very fussy about how its packets are transported. The details of how an IP packet is carried over a particular kind of network are usually chosen to be convenient for the network itself. As long as the transmitter and receiver observe some convention that allows IP packets to be differentiated from any other data that might be seen by the receiver, then IP can be used to carry data between those stations.

On a LAN, IP is carried in the data portion of the LAN frame and the frame header contains additional information that identifies the frame an an IP frame. Different LAN's have different conventions for carrying that additional information. On an Ethernet the Ethertype field is used; a value of 0x0800 identifies a frame that contains IP data. FDDI and Token Ring use frames that conform to IEEE 802 Logical Link Control, and on those LAN's IP is carried in Unnumbered Information frames with Source and Destination LSAP's of 0xAA and a SNAP header of 00-00-00-08-00.

The only thing that IP cares strongly about is the maximum size of a frame that can be carried on the medium. This controls whether, and to what extent, IP must break down large data packets into a train of smaller packets before arranging for them to be transmitted on the medium. This activity is called "fragmentation" and the resulting smaller and incomplete packets are called "fragments". The final destination is responsible for rebuilding the original IP packet from its fragments, an activity called "fragment reassembly".

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