Microsoft Knowledge Base Article 300435 contains the following summary:
This step-by-step instruction guide describes how Internet Security and Acceleration (ISA) Server 2000 publishes multiple Web sites to the Internet. This guide uses one server running on Windows 2000 Server SP2 with Microsoft Internet Information Services (IIS) and a second server running Windows 2000 Server Service Pack 2 (SP2) and ISA Server.
For people to find Web sites on the Internet, the site's name must be associated with the public IP address of the ISA Server computer's external network adapter. The ISP or network administrator can help with this. DNS will typically have an entry for www.your-domain-name.com pointing to the ISA Server computer's external adapter.