Hi, Janet,
If you haven't already gotten around this, you need to run UCX$CONFIG (@SYS$MANAGER:UCX$CONFIG).
It's menu-driven, so take option 1 (Core Environment) followed by Option 4 (Interfaces).
The configurator will interrogate your system and prompt you for new settings. When complete, you may need to restart your server (it's a while since I've had to do anything like that).
If your version of UCX doesn't use the configurator, you can go into UCX and use SET INTERFACE /NETWORK_MASK, but I'd be careful about this if it's something you're not used to. I generally get advice from my Compaq resident on stuff like that...
I'm using VMS 6.2 upwards, so I can't say if it's the same on 5.5, but I can't imagine it'll be too different.
Hope it works for you.
Jake