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Configuring ISA Server with a Single Network Adapter Configuration
Problem: There are a number of issues associated with the configuration of ISA Server on a computer with a single network adapter.
Cause: The causes include:


•Multi-network firewall policy. In single network adapter mode, ISA Server recognizes itself (the Local Host network). Everything else is recognized as the Internal network. There is no concept of an External network. The Microsoft Firewall service and application filters operate only in the context of the Local Host network. (ISA Server protects itself no matter what network template is applied.) Because the Firewall service and application filters operate in the context of the Local Host network, you can use access rules to allow non-Web protocols to the ISA Server computer. This has implications for running applications located on the ISA Server computer.


•Application layer inspection. Application level filtering does not function, except for Web Proxy Filter for Hypertext Transfer Protocol (HTTP), Secure HTTP (HTTPS), and File Transfer Protocol (FTP) over HTTP.

•Server publishing. Server publishing is not supported. Because there is no separation of Internal and External networks, ISA Server cannot provide the NAT functionality required in a server publishing scenario.

•Firewall clients. The Firewall Client application handles requests from Winsock applications that use the Firewall service. This service is not available in a single network adapter environment.

•SecureNAT clients. SecureNAT clients use ISA Server as a router to the Internet, and SecureNAT client requests are handled by the Firewall service. Because the Firewall service is not available in a single network adapter configuration, such requests are not supported.

•Virtual private networking. Site-to-site virtual private networks (VPNs), and remote access VPNs are not supported in a single network adapter scenario.

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So you are saying 2 different subnets would not cut it?