Hi Jonathan,
Windows 2003 Standard Edition IAS support up to 50 RADIUS client and 2 remote RADIUS server group.
You can define RADIUS client as IP Address or FQDN but not a range of IP Address or wildcard. In the case of FQDN when the RADIUS client resolves to multiple IP Address, the IAS uses the first IP Address returned in the DNS query.
Windows 2003 Enterprise and DataCenter Edition support unlimited number of IAS RADIUS client. You should upgrade.
Regards,
Dandy