The Internet Book by Comer Douglas E.;

Author:Comer, Douglas E.;
Language: eng
Format: epub
Publisher: CRC Press LLC
Published: 2015-07-15T00:00:00+00:00

17.5 Datagram Modification

The temporary IP address that a wireless router assigns to a device is not valid on the Internet. So how can a device with a temporary address communicate across the Internet? The wireless router modifies each outgoing datagram before sending the datagram on to the Internet. When a local device sends a datagram, the source address in the datagram will be the temporary address the device is using. When the datagram reaches the wireless router, software replaces the source address in the datagram with the valid IP address that the wireless router obtained from the modem. In essence, before a datagram goes out to the Internet, the wireless router replaces the “return” address with the valid IP address that the wireless router obtained when it powered on. When a host on the Internet replies, the reply will return to the wireless router.

If all outgoing datagrams have the same reply address, how can a wireless router know which of the local devices should receive a reply that arrives from the Internet? The wireless router keeps a list of what each device is doing (e.g., which web site is being accessed or which file is being downloaded). When a datagram arrives from the Internet, the wireless router consults its list to determine which device should receive the datagram. The wireless router replaces the “to” address in the datagram with the temporary address that has been assigned to the device.

NAT technology is complex, and you do not need to understand the details. The important point is:

By keeping a record of what each local device is doing and replacing addresses in incoming and outgoing datagrams, a wireless router allows local devices to communicate across the Internet simultaneously, even though the ISP has only assigned one valid IP address.

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