multiple NASes using just one RADIUS server

[Roy G Davis]

Title: multiple NASes using just one RADIUS server

hello
we are using freeradius for auth on a pix firewall.  right now it is just one firewall going to one radius server.  what would be the best way to add several firewalls (each of which would probably have a different set of users etc) to same radius server?  is this where REALMs would be used?  or would you run another instance of radius server listening on different ports?
TIA