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Is my NAT preventing connections from the outside?

On my Windows XP Pro home machine I have DSL Internet service that uses a Westell Versalink 327W modem/router which uses NAT (Network Address Translation).

I have read that if the NAT on a router is "dynamic" then the NAT acts as a firewall against attempts to connect to my machine from the outside. I would like to know if the NAT on my router is configured properly so that it is in fact preventing unwanted connections from the outside.

I have poked around the router config settings (careful not to change any) but I could not find anything that said clearly that the NAT was doing what I want.

How can I tell if my NAT is acting as a firewall?

Thanks, Billy