From scanning the booklet for Area 51, it says under 'Network Compatibility' that the in-game networking uses the official IANA registered PS2 UDP port number: ps-ams 3658/udp PlayStation AMS (Secure). It goes on to say that games will be unavaliable to you if the host is behind a NAT/firewall. After that it says Gamespy room pings require UDP Port 13139 to be open for unsolicited UDP.
Hmm.... I wonder if any of this would actually make many servers able to join
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