You might be questioning the inclusion of a section on virtual private network (VPN) technologies
in a chapter presenting cable modems. It is true that VPN is technology agnostic and will
operate over DSL, Frame Relay, or any other transport. However, cable modems and VPNs are
both covered briefly on the Remote Access exam, and neither seems to warrant a chapter on its
own. In addition, many cable modem installations for business customers leverage VPN tunnels
to provide connectivity.
A
virtual private network
is a logical tunnel across a physical topology. This physical layer
could be the Internet, or it could be a corporate network or other private network. The tunnel
need not be encrypted to be private, but this is a method of providing privacy. In reality, however,
so long as the data is not visible to non-recipients, the tunnel has a certain degree of protection.
As such, VPNs are commonly thought of as IPSec, L2TP (Layer 2 Tunneling Protocol),
SSL-VPN, and MPLS constructions, but Frame Relay and ATM PVCs, in addition to 802.1Q
and GRE (generic routing encapsulation) can also be considered VPNs. This is discussed in
greater detail later in this chapter.
By far the most common VPN technology deployed today is IPSec, or IP Security Protocol.
Quickly gaining momentum is an alternative technology that has been used for years for webbased
security, Secure Sockets Layer (SSL).