[CentOS] Easy way to reject all incoming packets except from certain IPs?

Is there an easy way to reject all incoming packets except those that come
from certain IPs? I can't find any way via iptables or via the GUI provided
with CentOS (or another GUI for CentOS) without having to resort to
Shorewall. It's fine if the answer is "go with Shorewall". I just didn't
want to have to become a Shorewall expert for this really small task.

Any help/advice on this is appreciated. Am I missing an easier way?

Preston
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