Because of its ubiquity, many companies now want to use a secure Virtual Private Network (VPN)
over the public Internet. The challenge in designing a VPN is to exploit the technologies for both
intracompany and intercompany communication while still providing security. Of course the rule of
thumb we now use in an Internet Protocol (IP) network is "IP on everything." A VPN is an extension
of an organization's private intranet across a public network (that is, the Internet), creating a secure
connection essentially through a tunnel. VPNs securely convey information across the Internet
connecting remote users, branch offices, and business partners into the corporate network. Figure
4−1 is a graphic depiction of an Internet−based VPN.