[jpstewart]

My take on whether or not you need a firewall on your IRIX box is that it depends on how much you trust the LAN that it is on. 

If it is a home LAN with only a small number of users, all of whom you trust, then there's no need for a firewall on every machine.  If you can't trust the rest of the LAN (e.g., corporate network) then yes you should run a firewall on every machine, especially your IRIX box.

Of course, the above assumes that you have a router & firewall that you trust between your LAN and the public Internet.

In my case, I have an ISP-provided router that I must use to connect to their network.  I don't know enough about it to trust its firewalling capabilities.  So I connect the ISP-supplied router to my own firewall.  (An old PC running up-to-date Linux can do the job, as vishnu points out.)  I can trust that firewall.  Then inside the firewall I have my private LAN where all my vintage computers can talk amongst themselves, isolated from the Internet.  Since everything on the LAN can be trusted, I don't care if NFS and Telnet are insecure by today's standards.  Nobody's snooping on my LAN inside the firewall without my permission.

So my advice is to read what everyone here is writing, but come up with your own plan customized your own needs and the trustworthiness of your LAN.