Not wanting to send product promotions to this list, I only replied
to the original poster. But since I see other traffic on this... As
one source of information, let me point people to TIS's home page
(www.tis.com) and the firewalls papers contained therein. There are
descriptions of what I called virtual network perimeter (what is being
referred to now as a Virtual Private Network).
Any firewall product, such as the Gauntlet Firewall, that supports
firewall to firewall encryption can be used to set up a VPN. The
requirement (and this is mentioned in one of my papers I think) should
be obvious but often is not: if you are going to extend your network
security perimeter to include other networks, they'd better share the
same security policy, security posture, and administrative domain
(management chain for example, not domain in the DNS sense).