Port Forwarding

[ekay]

So if you want to play your own missions in co-op then you'll need to host your own game. Which means you'll have to forward ports on your router.

PortForward.com is the best place to go.
Select your router make and model, select Borderlands, find out your IP address.

Ports you need to forward are:
7777 (TCP and UDP)
3783 (Voice Chat Port)
27900 (Master Server UDP Heartbeat)
28900 (Master Server List Request)
29900 (GP Connection Manager)
29901 (GP Search Manager)
13139 (Custom UDP Pings)
6515 (Dplay UDP)
6500 (Query Port)

To find your IP address open the command window and run IPCONFIG

[Zofo]

Is forwarding ports dangerous?  This was the main solutions to my problems with Section 8

[Paranoia]

Not at all, Zofo.

[zimnim]

i got away with just

 7777 tcp & udp
28900 tcp
27900 udp
28910 tcp

28902 tcp ( this one was not essential but did resolve some people not being able to join)

[ekay]

Yeah, those are the ones I used that PortForward.com suggests. I got the full list of all the ports from the Steam forums and thought there is no harm in forwarding all of them.

@Zofo; I've heard some people just opening ALL ports, which I guess is the lazy way to do it. You could also just open ports from 3783 through to 29901 to cover all of them which would also work, but if you want to be a little more cautious then just open the required ones. (It's a time:effort ratio thing)