Suppose you have a corporate webservice that you want to call from outside. The external party should not be aware of the internal server names. That’s why the external parties call the service by an alias defined on the ISA Server. As an example the alias can be test-company.nl. The ISA server is responsible for translating the alias to the internal DNS name, BizTalkServer in the picture below. The technical name for this naming transition is Network Address Translation (NAT).
Translating the DNS name to an IP address is the responsibility of the DNS server. It’s important to note that both BizTalkServer and the alias test-company.nl map to the same IP address. As a result, both internal parties and external parties can call the service by the alias. That means in turn that you can change the actual BizTalkServer, without impacting the service clients.