Configuration of Norton Intenet Security

I am trying to configure Norton Internet Security 2003 -2005 on a few workstations. I was wondering what is more secure. I want to configure the firewall to trust the other computers on my network. Which is better, when the wizard detects a non routable address, it adds the whole subnet that this address belongs to the trusted zone. Or to manually add the IP addresses. I was reading about IP-spoofing and was wondering if one method is better than the other to defend this…