this is what i got from a naive search
so i think the ports assumption might be valid.
i haven’t played all the games on steam, so i can’t verify that claim.
but when i said open ports i meant on the server. the player usually don’t need them.
but the server almost always does need them.
the problem is games that are listen-servers and it’s player to player. at that point you’ll need tunneling like hamachi or smth. and i don’t know if steam does or doesn’t provide it (and cant recall eos).