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What is the purpose of having 3 Ethernet ports on a router/firewall like this one: vs. just having 2 like this:

What can you do with 3 vs just 2?

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up vote 6 down vote accepted
  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.

WIkipedia DMZ diagram

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Ah, the siege warfare scenario of IT protection, bravo @RedGrittyBrick :P – zackrspv Jan 4 '12 at 22:35

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.

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