Any service or server that acts as a proxy for client computer requests at the application’s protocols.
For example, in Microsoft Proxy Server, the Web Proxy Service is an application layer proxy for the Hypertext Transfer Protocol (HTTP), Secure Hypertext Transfer Protocol (S-HTTP), File Transfer Protocol (FTP), and Gopher protocols. Application layer proxies provide security by hiding internal network addresses from the outside world.
Application layer proxies provide more support for the additional capabilities of each protocol than do circuit layer proxies. For example, application layer proxies can support virus scanning. Application layer proxies are also client-neutral and require no special software components or operating system on the client computer to enable the client to communicate with servers on the Internet using the proxy server.
Microsoft Proxy Server can grant users access to selected application layer protocols and can restrict access to remote Web sites by domain name, IP address, and subnet mask.