I don't remember the old DEC unix too well. Under /etc/hosts is the table of IP addresses you should be able to edit that for this machine. It might look something like this...
$ cat /etc/hosts
#
# Internet host table
#
127.0.0.1 localhost
192.168.0.209 Your.machine.com myunix loghost
172.16.25.12 other.machine.com someotherbox loghost u

There might be other os file. You would have to get a Dec manual to check.