We recently installed a IPFW firewall. Blocking IP's is no problem, but how can I block something like 227-216.205.94.dellhost.com I can't resolve the IP and adding 227-216.205.94.dellhost.com caused a reboot of the server.

If you can't resolve the FQDN to an IP then you can't block it. But you would not see that if it did not (at least for some period of time) resolve to an IP.

IPTables/IPChains firewall work with IPs, you can specify the hostname in many IPTables/IPChains based firewall however they are resolved to actual IP addresses at runtime.

How should I block things like 227-216.205.94.dellhost.com This account has already been cancelled by Dellhost because of abuse. Otherwise I wouldn't have mentioned it.

There must be a way to resolve the FQDN to an IP because the IP is doing(together with the MAC) the communication. According to the OSI layers.

Even with spoofing you must use an (fake)IP.

Anyone any idea how this is done?