An application level gateway allows computers on a local area or “internal” network to serve data (e.g., web pages, files or other constructs) to computer systems on an external or public network such as the Internet, even though references such as hostnames and/or network addresses within the internal network that are contained within the data (e.g., URLs in web pages) might not be compatible (e.g., DNS resolvable or routable) with the external network. The system detects, in a portion of data (e.g., a web page), a local reference to a computer system on the internal network, determines whether a computer system identifier is mapped to the computer system specified in the local reference, and replaces the local reference with a translated reference obtained from the mapping. The translated reference contains the computer system identifier and a reference to a gateway computer system coupled to the internal network, such that subsequent referrals to the translated reference are directed to the gateway computer system. When a request for the data is subsequently received, the gateway performs a reverse mapping to determine the identity of the computer system on the internal network.