What can you use three ethernet ports for on a firewall/router?

Question

What is the purpose of having 3 Ethernet ports on a router/firewall like this one: http://store.netgate.com/Netgate-m1n1wall-2D3-2D13-Red-P218C83.aspx vs. just having 2 like this: http://store.netgate.com/Netgate-m1n1wall-2D2-Red-P221C83.aspx

What can you do with 3 vs just 2?

Answer 1

1. The outside world - nasty. bad. dangerous.
2. The inside world - soft, nice. protected.
3. The DMZ - where we put our trembling web server for outsiders.

If the DMZ is stormed, we are still cozy on the inside.

Answer 2

One of your ethernet ports is for connecting to the outside world so if you have three ports rather than two on your machine, you can have two internal networks.

This is probably most useful if you have a webserver or other public facing machine - by having it on a separate network from your truly internal machines, it makes it less likely that anybody managing to do bad things to your webserver can do bad things to your internal machines.

See this Wikipedia article on DMZs.