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Network address translation (NAT) is a methodology of remapping one IP address space into another by modifying network address information in Internet Protocol (IP) datagram packet headers while they are in transit across a traffic routing device. The technique was originally used for ease of rerouting traffic in IP networks without renumbering every host. It has become a popular and essential tool in conserving global address space allocations in face of IPv4 address exhaustion. ==Methodology== The original use of network address translation consisted of mapping every address of one address space to a corresponding address in another space, such as when an enterprise changed Internet service providers without having a facility to announce a public route to the network. In the face of the foreseeable global IP address space exhaustion, NAT was increasingly used since the late 1990s in conjunction with IP masquerading, which is a technique that hides an entire IP address space, usually consisting of private network IP addresses (RFC 1918), behind a single IP address in another, usually public address space. This mechanism is implemented in a routing device that uses stateful translation tables to map the "hidden" addresses into a single IP address and readdresses the outgoing IP packets on exit so they appear to originate from the routing device. In the reverse communications path, responses are mapped back to the originating IP addresses using the rules ("state") stored in the translation tables. The translation table rules established in this fashion are flushed after a short period unless new traffic refreshes their state to prevent port exhaustion and free state table resources. The method enables communication through the router only when the conversation originates in the masqueraded network since this establishes the translation tables. For example, a web browser in the masqueraded network can browse a website outside, but a web browser outside could not browse a website hosted within the masqueraded network. However, most NAT devices today allow the network administrator to configure translation table entries for permanent use. This feature is often referred to as "static NAT" or port forwarding and allows traffic originating in the "outside" network to reach designated hosts in the masqueraded network. Because of the popularity of this technique to conserve IPv4 address space, the term ''NAT'' has become virtually synonymous with the method of IP masquerading. As network address translation modifies the IP address information in packets, it has serious consequences on the quality of Internet connectivity and requires careful attention to the details of its implementation. NAT implementations vary widely in their specific behavior in various addressing cases and their effect on network traffic. The specifics of NAT behavior is not commonly documented by vendors of equipment containing implementations. 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「network address translation」の詳細全文を読む スポンサード リンク
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