I can't find a good site describing it, so I'll give it a try:

You have 1 router that is connected to the internet with a public IP address. (call it http://www.myserver.com) When users on the Net try to connect to your router (www.myserver.com), the router on your LAN will redirect that traffic to a server inside of your LAN. This is done transparantly to the client.

Say a user on the net goes to http://www.myserver.com, port 80 (standard HTTP port). Your router takes the request and examines the port. You would have previously configured your router to send all port 80 traffic to a specific INTERNAL IP address, which would be a web server.

You can also use a DMZ (demilitarized zone), I found a decent explanation here:
http://www.nfr.net/firewall-wizards/.../Jan/0067.html


here is a good page here as well
http://msdn.microsoft.com/library/te...ablueprint.htm