1. NAT 타입 1
- 유선랜 직접연결, 최상의 방법으로, 통신업체의 모뎀이나, 벽에서 부터 직접 연결시 받게됨 (Real IP 사용시)

2. NAT 타입 2
- 유선랜 라우터를 통한 연결, 이경우 라우터의 UDP 포트 3658번 열어주고, UPnP를 사용가능하게 변경함으로써 보다나은 환경으로 게임 가능
- 무선랜 라우터를 통한 연결,
- 포트 오픈을 위해서는 각 라우터의 Port Forwarding 기능을 이용

3. NAT 타입 3
- 유. 무선랜의 라우터를 통한 연결, 네트워크 환경 개선을 위해서는 세팅 설정이 다소 필요함
- 옵션 메뉴의 UPnP 세팅 메뉴의 모드를 1 혹은 2로 변경
- UPnP 세팅 변경만으로도 부족할 경우, 포트 포워딩 기능을 사용하여 개선할 수 있음

% GT5P 온라인에서 사용 하는 포트 %
TCP Ports : 80, 443, 5223, 10070 - 10080
UDP Ports : 3478, 3479, 3658, 10070 - 50100

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북미쪽 공식홈에 뜬 내용입니다.
요즘엔 그란의 넷환경이 많이 좋아져서 별 문제가 없습니다만, 필요하신 분은 참고하시면 될듯,
추가 설명을 하자면,

TCP 포트중 80은 http(웹페이지형식)를 위한것 같고, 443 은 SSL보안인증을 하기위함으로, 패치 다운로드시 사용할것 같네요,
5223 과 10070 은 온라인 게임중 데이타를 주고받는 포트 같습니다. UDP 쪽은 잘 모르겠습니다. ^^;

저는 NAT 2 로 나오고 있습니다만, NAT 1 으로 하실려면, 라우터의 DMZ 서버 기능을 이용하시면 됩니다.
NAT 2 로해도 요즘엔 호스트 잘잡고, 게임하는데도 무리는 없는거 같네요.

참고들 하세요.

- 이하 원문 -

The online racing mode in Gran Turismo™ 5 Prologue offers a multitude of races and events for players of all skill levels and cars of all classes. Up to 16 people can compete in a single race, meaning there's always someone new to challenge. In order to keep the online experience an optimal one for everyone, the game automatically evaluates the quality and speed of the players’ connections and matches them with an appropriate number of opponents, up to 16-player races for the strongest connections, while still maintaining optimal networking performance for everyone in each race.

There are several ways players can optimize their network environment to squeeze the best performance out of their connections, just like tweaking engine settings for maximum horsepower output. Below are hints and tips for maximizing the performance of your online connection for the best online racing experience. You can also visit our official forums to chat with other users for network equipment-specific questions and tweaks.

Gran Turismo 5 Prologue auto-detects your network connection configuration, displayed in the game by assigning it a “NAT Type,” and matches you up with other players to give you the best online race experience possible for your connection quality.

Wired connection from a modem (NAT Type 1)
When playing Gran Turismo 5 Prologue online, the best way to connect your PLAYSTATION?3 system to the internet is to plug it directly into your broadband modem.


We know that gamers are on the cutting edge and may have multiple devices connected to the internet at once through routers or other devices, so in case a direct connection isn’t an option, here are a few other setup options available to you and tweaks you can use to maximize the online performance of the game.

Wired connection through a router (NAT Type 2)
Connecting your PLAYSTATION?3 system to a router with an Ethernet cable is another way to take your game online if a direct connection to the modem is not possible.

If this applies to you, opening UDP Port 3658 on your router/modem with UPnP enabled can help make for a more stable online racing experience. Your router instruction manual or manufacturer will be able to assist you with opening ports or "Port Forwarding."
  
Wireless connection (NAT Type 2)
If you want to take advantage of the PLAYSTATION?3 system’s wireless networking for Gran Turismo 5 Prologue, just configure your router to have UDP Port 3658 open on your wireless router/modem with UPnP enabled for a more stable online racing experience. Your router instruction manual or manufacturer will be able to assist you with opening ports or "Port Forwarding.”


Wired or Wireless Connection through a router (NAT Type 3)
If the game displays your NAT Type as NAT Type 3, there are some settings you can tweak to improve online performance and your connection quality with other players in the game.

In the Options menu for Gran Turismo 5 Prologue, change the UPnP settings to see if you can acquire a better connection. You can select Mode 1 or Mode 2.

If adjusting UPnP doesn’t provide enough of a boost, then you may need to open the network ports listed below to achieve better online connectivity. Your router instruction manual or manufacturer will be able to assist you with opening ports or "Port Forwarding.”
Gran Turismo 5 Prologue online uses the following ports for communication with game servers. If you experience problems connecting to online races, you may want to check the status of these ports in your router or firewall to see if they are being blocked or filtered. Your router instruction manual or manufacturer will be able to assist you with opening these ports up.
• TCP Ports: 80, 443, 5223, and 10070 - 10080

• UDP Ports: 3478, 3479, 3658, 10070, and 50100

[Note: Make sure to enable these TCP/UDP ports in BOTH directions and are entered into the TCP and UDP port fields in your router / modem.]