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NAT traversal is a computer networking methodology with the goal to establish and maintain Internet protocol connections across gateways that implement network address translation (NAT). NAT breaks the principle of end-to-end connectivity originally envisioned in the design of the Internet. NAT traversal techniques are required for certain client-to-client network applications, such as peer-to-peer file sharing and Voice over IP. == Explanation == Many techniques exist, but no single method works in every situation since NAT behavior is not standardized. Many NAT traversal techniques require assistance from a server at a publicly routable IP address. Some methods use the server only when establishing the connection, while others are based on relaying all data through it, which adds bandwidth costs and increases latency, detrimental to real-time voice and video communications. Most NAT behavior-based techniques bypass enterprise security policies. Enterprise security experts prefer techniques that explicitly cooperate with NAT and firewalls, allowing NAT traversal while still enabling marshalling at the NAT to enforce enterprise security policies. IETF standards based on this security model are Realm-Specific IP (RSIP) and middlebox communications (MIDCOM). NAT devices are commonly used to alleviate IPv4 address exhaustion by allowing the use of private IP addresses on home and corporate networks behind routers with a single public IP address facing the public Internet. The internal network devices communicate with hosts on the external network by changing the source address of outgoing requests to that of the NAT device and relaying replies back to the originating device. This leaves the internal network ill-suited for hosting servers, as the NAT device has no automatic method of determining the internal host for which incoming packets are destined. This is not a problem for home users behind NAT devices doing general web access and e-mail. However, applications such as peer-to-peer file sharing, VoIP services and the online services of current generation video game consoles require clients to be servers as well, thereby posing a problem for users behind NAT devices, as incoming requests cannot be easily correlated to the proper internal host. Furthermore, many of these types of services carry IP address and port number information in the application data, potentially requiring substitution or special traversal techniques for NAT traversal. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「NAT traversal」の詳細全文を読む スポンサード リンク
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