Is my NAT preventing connections from the outside?

Question

On my Windows XP Pro home machine I have DSL Internetservice that uses a Westell Versalink 327W modem/routerwhich uses NAT (Network Address Translation).I have read that if the NAT on a router is "dynamic" thenthe NAT acts as a firewall against attempts to connect tomy machine from the outside.  I would like to know if theNAT on my router is configured properly so that it isin fact preventing unwanted connections from the outside.I have poked around the router config settings (careful notto change any) but I could not find anything that saidclearly that the NAT was doing what I want.How can I tell if my NAT is acting as a firewall?Thanks,Billy

Duane Arnold · Accepted Answer

"Billy Smith" wrote in news:BTzLe.5856$rR4.4530 @trnddc08:

The NAT router does that by default by not forwarding unsolicited traffic to the LAN. In other words, if a machine behind the router didn't make a solicitation to a remote IP on the Internet, the inbound traffic is dropped. If a machine behind the router made a solicitation to a remote IP, then the traffic is allowed back to the machine.

NAT software is not FW software and some say it is but I for one don't agree that it's FW software. But by not forwarding requests to the LAN, NAT does protect the LAN in a limited fashion.

However, the simplest form of a FW is a router that has a public facing interface and WAN and private facing interface the LAN and separates two networks which is usually the Internet (WAN) the network it is protecting from and the (LAN) the machines behind the router connected to it and is protecting them.

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The link above has other topics like what is Port Forwading and the DMZ if the router has a DMZ.

The two links may help you to understand FW technology a little better.

Duane :)

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Alt Beer · Answer

You could try doing a scan of Common Ports and All Service Ports at GibsonResearch web site.  It will indicate whether your ports are visible toothers.

Billy Smith · Answer

Thanks, your responses are helpful.--Billy

NormanM · Answer

NAT is not a firewall, though NAT devices which block unsolicited packets are acting like firewalls. AFAIK, your NAT router is capable of blocking unsolicited packets. As somebody else has suggested, a port scanning site, such as

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will show you if packets are being dropped, or passed.

Before somebody jumps in on my statement about NAT and firewall, I have set up a hardware NAT device which did not block any unsolicited packets, as tested at

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an Efficient Networks SpeedStream 4100, as shipped by SBC for their SBC Yahoo! DSL Service Express package. A low end device, the SS4100 does NAT, assigning an RFC 1918 IP address to the computer; but it definitely passes unsolicited packets to the computer. Nor does it have a DHCP service, so it is not a router, either. Just a DSL modem which performs PPPoE, and NAT to a single IP address.

Leythos · Answer

NAT does not mean that inbound is always rejected, in fact there are 1:1 NAT, Many:Many NAT, and 1:Many NAT setups depending on the device. Just because a device has NAT does not indicate it's blocking anything - some NAT setup pass all traffic inbound.

Is my NAT preventing connections from the outside?

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