The key step in the Layer 3 routing process is a network address table lookup. A router maintains a table that matches address prefixes to output ports.
The specific processing steps that are carried out differ slightly for each of the network-layer protocols. A rough outline that includes most of the steps performed by an IP router follows:
After discarding the frame header and trailer, the datagram is passed to an IP module for processing.
The IP header contains a checksum field. The checksum is recalculated, and the datagram is discarded if the outcome does not match the value in the field.
The IP header includes a field that limits the number of hops that a datagram may traverse. This is decremented, and if the value is ...