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The Router

The router is used to route packets from one LAN (or any network) to another. It is a layer 3 device where the IP protocol is used. Each packet has a source and destination IP address. The router looks at each incoming packet's destination IP address and decides which outgoing interface to send it out on. An IP packet will travel from router to router until finally arriving at its destination.

Routers look up routes in their routing tables to decide where to route the packet. Routers build these tables by talking to other routers using routing protocols. Some common routing protocols are RIP, EIGRP and OSPF.

A router usually can do a lot more than routing. It can provide DHCP (Dynamic Host Configuration Protocol) services, NAT/PAT (Network Address Translation/Port Address Translation) services and ACL (Access Control Lists) for starters. DHCP simplifies the configuration of PC's. NAT/PAT allows large numbers of private addresses to be used on LANs which are then translated to one or a small pool of public IP addresses for use over the Internet. Example private addresses are 192.168.x.x and 172.16.x.x. NAT/PAT provided a good solution to the overall shortage of IP addresses worldwide.