I keep on getting disconnected from Alien Swarm (after 10 retries the game auto disconnects) and I remembered that I have very few ports forwarded to my computer.

What ports should I forward to my computer so that I can play most effectively?

2 Answers 2


I believe 27015 is the port you should be forwarding for client connection. A server can be hosted on any port, but this appears to be the default one.

If you want to host a dedicated server, you would need to forward 27005 as well.

Alternatively, this post on Steam forums suggests enabling

  • UDP 27000 to 27020 inclusive
  • TCP 27020 to 27050 inclusive
  • So, I think those are the right ports, but I can't tell, because I'm having connection difficulties still. Any suggestions?
    – McKay
    Commented Jul 26, 2010 at 15:53

I have seen a post on Steam forums which says that you need ports 27005, 27015, 27020, and 49872. When you forward the ports in range 27000-27050, this covers the first three of this list, but not the last one.

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