One of the primary limitations of the ISA Server 2000 firewall was its simplistic view of the network. The ISA Server 2000 firewall recognized only two types of networks: trusted and untrusted. Trusted networks were included in the ISA Server 2000 firewall s Local Address Table (LAT). Any network that wasn t in the LAT was considered untrusted. ISA firewall policy was applied to all communications between LAT and non-LAT hosts. Communications between LAT hosts were routed through the ISA Server 2000 firewall without being subjected to the ISA Server 2000 firewall s stateful filtering and application-layer inspection mechanisms.

This was problematic for ISA Server 2000 firewall administrators who wanted to create DMZ segments that were directly connected to the ISA Server 2000 firewall. For example, an ISA Server 2000 firewall might be configured with three network interfaces. This configuration could include an internal interface connecting to the internal network, a DMZ interface connected to a public access DMZ segment, and an external interface, which connects the firewall to the Internet.

In ISA Server 2000, this trihomed DMZ configuration highlights most of the limitations of the ISA Server 2000 networking model.

• All communications between LAT and non-NAT hosts had to be NATed. This meant that all connections between the internal network and the Internet, and the internal network and the DMZ segment, were NATed.

• The ISA Server 2000 firewall did not apply stateful application-layer inspection to connections between Internet hosts and machines on the DMZ...