Network Address Translation

By Pete Loshin, Computerworld |  Networking

Just ask any economist: When a commodity is in short supply, several things happen. The price goes up, rationing begins, and people start scrambling for substitutes. Globally unique Internet addresses, usually called Internet Protocol (IP) addresses, are no exception.

The Internet Engineering Task Force has been aware of the impending depletion of the current address space, called IPv4, for almost a decade. Although the forthcoming IPv6 is still seen as the long-term solution for continued Internet growth, other short-term fixes have been institutionalized in the past several years.

RFC 1631, "The IP Network Address Translator," published in 1994, describes one such fix. In the early days of the Internet, people were urged to apply for globally unique network addresses regardless of whether they ever intended to connect to the global Internet. The idea was to avoid problems when a formerly private network was eventually hooked up to the public Internet.

As the Internet continued to grow exponentially, however, assigning perfectly good network addresses to private networks came to be seen as a waste of valuable virtual real estate. Under the Network Address Translation (NAT) standard, certain IP addresses are set aside for reuse by private networks. As specified in RFC 1597, "Address Allocation for Private Internets," anyone can use addresses in the following ranges: 10.0.0.0 to 10.255.255.255; 172.16.0.0 to 172.31.255.255; and 192.168.0.0 to 192.168.255.255. By convention, routers aren't supposed to forward any packets to these addresses on the Internet.

The simplest NAT device has two network connections: one on the Internet and one on the private network. Hosts within the private network, using their private IP addresses (sometimes also called Network 10 addresses, from the 10.0.0.0 address set aside for private use) connect to the Internet by sending packets directly to the NAT device. Unlike normal routers, which merely read the source and destination addresses on each packet before forwarding them to their destinations, NAT devices actually modify the packet headers, changing the private network source address into its own Internet address.

NAT Drawbacks

In using NAT, hosts on the Internet appear to be communicating directly with the NAT device rather than with the actual host inside the private network. Inbound packets are sent to the NAT device's IP address, and the device changes the destination packet header from its own Internet address to the private network address of the true destination host.

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