Spoofing/Bridging? Confused

[rwinegar]

I have DSL. I have two computers connected to a Linksys BFSR41 router. The router is connected to a Thompson modem which is connected to the Internet. My ISP uses PPPOA authentication. The modem has a NAT firewall and does the dialing. My router is set to obtain an IP address automatically. With my router set to obtain an IP automatically, I believe it is merely acting as a switch. My question is this - is the NAT firewall in my router still functioning? What about the NAT firewall in my modem? Can both firewalls work simultaneously?

Thanks.

[104456]

NAT serves three main purposes:

Provides a type of firewall by hiding internal IP addresses
Enables a company to use more internal IP addresses. Since they're used internally only, there's no possibility of conflict with IP addresses used by other companies and organizations.
Allows a company to combine multiple ISDN connections into a single Internet connection.

http://www.webopedia.com/TERM/N/NAT.html

Modems in themselves dont have NAT that I know of.Routers have NAT to provide translation but it is not a firewall per se as say software or hardware firewalls which are designed to allow or block traffic to a preconfigfured set of rules.

[rwinegar]

Thanks 104456. My Thompson modem definately has a NAT firewall. What I need to know is when the Linksys router is used as a switch (set to obtain IP automatically), is the routers NAT firewall enabled.

[DrMDJ]

When the router is set to obtain an ip address from you isp (as opposed to when your isp gives you a permanent ip address) it has nothing to do with whether the nat capabilities of the router are being used. The router uses NAT in regard to your LAN conencted PCs and their IP addresses, and it would still use nat to mask these ip addresses from the outside (WAN/internet) world.

[rwinegar]

Thanks DrMDJ. That makes sense and is the answer I was looking for. So I've got two NAT firewalls - one in my modem/router (single port) and one in my Linksys router.

[Train]

But to be safe, get a software firewall. Yes I have been hit through my router many a time.