The key ingredient in VPN is security. VPN allows you to use a public network
with a high degree of certainty that the data sent across it will be secure. You can think
of VPN as a secure pipe through the Internet connecting computers on either end, as
you can see in Figure 10-1. Information is able to travel through the pipe securely and
without regard to the fact that it is part of the Internet. This concept of a pipe though the
Internet is called tunneling. The secure ???tunnel??? is achieved by first encrypting the data,
including all its addressing and sequencing information (where the individual piece of
data, called a ???datagram,??? fits in a longer message), and then encapsulating or wrapping
that encrypted data in a new Internet Protocol (IP) header with routing and addressing
information, as shown next. The outer package can then weave its way through the
servers and routers of the Internet without the inner datagram ever being exposed, and
should it ever be, it is still encrypted.
VPN replaces both leased lines between facilities and the need for long-distance dialup
connections. For example, before VPN a company needed a leased line between the
headquarters and a branch office in another city. With VPN, each office just needs a local,
probably high-speed, connection to the Internet, which is then used with VPN to
securely transmit information between the offices. In another example, before VPN a
traveling worker would make a long-distance call into a remote access server to connect
to the company??™s LAN, incurring a long-distance charge.
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