Chapter Summary


Routing starts with a host computer that has a packet to send. The host computer starts by deciding whether the packet destination is on a different subnet. If the destination is on a different subnet, the host sends the packet to the default gateway. If the destination is on the same subnet, the host sends the packet directly to that host, ignoring the default gateway.

Before the host can send a packet over an Ethernet NIC, it must know the destination MAC address of the device. The host looks in its ARP cache, which is a list of IP addresses on the same subnet, along with the corresponding Ethernet MAC addresses. If there are no entries, the host uses ARP to broadcast a request for the information, expecting to get a reply that includes the missing information.

A router receives Ethernet frames addressed to it, and if the frame is free from errors, the router de-encapsulates the IP packet from inside the frame. The router compares the destination IP address of the packet to the routing table entries. The entry that the router matches tells the router where to send the packet next. Those forwarding instructions include the router interface out which to forward the packet, called the outgoing interface, along with a next-hop router, if appropriate. This basic logic works whether or not subnetting is in use.




Computer Networking first-step
Computer Networking First-Step
ISBN: 1587201011
EAN: 2147483647
Year: 2004
Pages: 173
Authors: Wendell Odom

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