Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

The proxy server can be viewed as a gateway between two networks, usually a private internal network and the Internet. It is sometimes called an application-level gateway. The proxy server hides the internal network from the external network. It keeps hackers from accessing or even knowing about internal IP addresses.

A proxy server runs as software on a computer and acts on behalf of a client to make requests outside the client's network. For example, when an internal user attempts to access the Internet, the proxy server intercepts the request and makes the request itself. The internal user never makes a direct request to an outside system. Likewise, when the Internet server returns a response, it is intercepted by the proxy server and transferred to the user. The proxy server can filter all incoming packets and discard any that are not related to an internal request. This prevents hackers from attacking internal systems.

Proxy servers are both firewall and caching systems. Since they provide a centralized location where internal users access the Internet, the proxy server can cache frequently accessed documents from sites on the Internet and make them quickly available for other internal users that need the documents.

This topic is covered in more detail under "Firewall."

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