playing online with a firewall

i have a firewall on my computer and i was wondering if any one new how i could play online with it still active do i have to open ports or somthing

No. Why dont you google things before posting here.

My honest suggestion is switch over to microsoft security essentials but because its unlikely you will, I’ll just say find the ports gmod uses or allow gmod as an excpetion

thanks for your help unlike flamehead 5

I use Comodo, why would it be an issue though?

Always glad to help, and @ j!nx; alot of firewalls are rather crappy an just simply block all non-browser programs trying to use the internet, if someone has one of those kinds of firewalls, the only thing you can do to make that firewall accept it is to either make an exception (if the firewall allows exceptions) or force the firewall to not cover the ports used for whatever your wanting to do.