Application Layer Gateway (ALG)
An ALG is a security component that augments a firewall or NAT employed in a computer network. It allows legitimate application data to pass through the security checks of the firewall that would have otherwise restricted the traffic for not meeting its limited filter criteria.
The need for IP Address translation arises when a network's internal IP addresses cannot be used outside the network either because they are invalid for use outside, or because the internal addressing must be kept private from the external network.
NAT devices attempt to provide a transparent routing solution to end hosts trying to communicate from disparate address realms. The NAT function cannot by itself support all applications transparently and often must co-exist with application level gateways (ALGs) for this reason.
People looking to deploy NAT based solutions need to determine their application requirements first and assess the NAT extensions (i.e., ALGs) necessary to provide application transparency for their environment. - copyright (C) The Internet Society (1999); All Rights Reserved
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