Netscape uses a lock file to prevent corruption of its internally-used files that would happen when two copies of the program are running at the same time. This file is stored in the user's home directory under
I don't have an old enough system to test this out for myself, but Firefox, which adopted many of Netscape's habits, creates a lock file that's a symbolic link to a nonexistant file that contains the IP address of the host that created it and the process ID, like this:
% ls -lh .mozilla/firefox/*/lock
lrwxrwxrwx. 1 blrfl blrfl 17 Aug 23 20:33 .mozilla/firefox/angfgbte.default/lock -> 192.168.1.111:+1234
In the era when the Indy was built, it was common to have home directories served up to the workstations across the network so users could log into any of them and get at their files. Storing the IP address where the first instance of Netscape was running would let a second instance inform its user where the first was running.
What likely happened in your case is that the machine was running Netscape when it was last shut down or lost power before you got it, leaving the lock file in place. (I don't remember if Irix of that vintage neatly terminated running processes during shutdown.) When you started Netscape back up, it had no way of knowing that it wasn't on the same network, saw the lock file, refused to start and gave you the best information it had about where the other process might be running.
The way to solve the problem is to remove
~/.netscape/lock and start Netscape.