Sorry I missed your comment earlier that the Westell is indeed a NAT router in addition to being a modem. So you should not need to buy another NAT router. Nor should you need to configure your windows or linux box as a router.

But if you are getting ISP-issued, as opposed to locatl, IP addresses, there must be something wrong with your NAT configuration. Perhaps your Westell is configured to behave like a simple modem with its NAT/router facility disabled.

Look in the manual for your modem/router - if you've lost it there's one on-line at www.westell.com under "support." You should be able to do this with a web browser from either computer. Then tell it to connect and make sure it has NAT enabled. Sorry to have given you vague advice in the past - it is really difficult in this forum to communicate the minute details (in both directions).