It seems odd at first, but you add multiple agents to the server, each with their own unique name but with the same CIDR network address. OSSEC sorts them out by name just fine with the two DHCP clients I'm testing.
Only potential hiccup there is if you have a real "road warrior" on a laptop who roams from outside your LAN, he'll be attaching with an address outside the normal DHCP range. Haven't tested that case....yet. Good luck, Dave
