As the IP software constructs each IP packet, the IP software passes each packet to the
data link software, which in most cases today means the driver for the network interface. However, if the
destination IP address is not on the local network, the command to the data link layer tells the data link layer
to send the message to the local router, or gateway, computer.
The data link software will build an appropriate data link header of its own, whose format depends on the
type of data link, and write the bits to the destination computer. At this point, the destination computer is either
another computer on the local network, or a router attached to the local network.
When the IP packet arrives at the router, the router??™s data link software will remove the data-link-level
header, and forward the IP packet to the IP software. The IP software in the router will check the IP header
checksum, and consult its tables to decide where to forward the message in order best to move the
message toward its destination. The IP software will pass the IP packet back to the data link layer of the router,
with instructions to send the packet over the adjoining network to the appropriate computer, or to the
next router.
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