Forum OpenACS Q&A: Response to why cannot see the pages outside of my machine?
A little more trivia about the ACS 3.x nsd.tcl file:
At least in 1998-99, ArsDigita typically used servers with multiple IP addresses. Machines were used to host several sites, typically a production site and a development site; other sites on the same machine also included a document or staff site and maybe a staging site. Each ACS instance was tied to a separate IP address. Some aD machines actually hosted several different client sites, but maybe no dev sites. They did not use virtual host redirecting. I have no idea what aD does nowadays.
At least in ACS 3.x, there's a lot of legacy cruft. I don't know anything about ACS 4.x/OpenACS 4.x since I've never looked at it.