Ports are extensions to IP addresses used to define the type of information being transmitted. Packets sent or received using port 80, for example, will, by convention, contain data being used to surf websites using the HTTP protocol, while port 25 is used to handle SMTP email messages, port 110 for Pop3 message retrieval and so on.

On most routers, therefore, you can configure rules to forward all port 80 traffic to your local web server and direct port 25 and 110 traffic to Exchange or whatever email server you're running. Exactly how you go about configuring these rules will depend on the router involved, but in most cases it's simply a matter of choosing the type of traffic or port to be forwarded, then choosing a host from a list of those found on the Lan.

One of the advantages of port forwarding is that it lets you get away with having only one public IP address and it doesn't have to be fixed. However, if you've paid for additional addresses to assign your servers, some routers will also let you direct all the traffic sent to that address to a local system behind the Nat firewall.