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?
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
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.