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equired) can be sent across public backbone networks in a secure manner. VPNs
offer tremendous cost savings to companies that need private connectivity between
offices. See Figure 9.3 for a point-to-point tunneling topology example.
Figure 9.3. A point-to-point tunneling example.
There are two types of VPNs: encrypted and unencrypted. Unencrypted VPNs
encapsulate data inside of a tunneling protocol packet. With an encrypted VPN, the
protocol datagram reaches the VPN gateway, where it is encrypted and placed
inside another protocol datagram for delivery. When the datagram reaches the
destination router, the packet is decrypted and forwarded on to its destination host.
Several open and private VPN encryption standards exist. Secure Shell (SSH),
Point-to-Point Tunneling Protocol (PPTP), SOCKS, and IPSec are all popular secure
tunneling methods. All these tunneling approaches use some combination of the
40-bit data encryption standard and RSA public key encryption. The Cisco IOS
supports VPNs through both encrypted (DES-40, DES-56, DES-128-bit) and
unencrypted tunnels.
NOTE
DES is a private key crypto algorithm based on a 56-bit encryption key, in which
both the sender and receiver share a common key. RSA is a public key encryption
system in which two keys are used: a public key used to encrypt data, and a private
key used to decrypt data. There are no specific minimum and maximum RSA key
lengths. However, most applications use key sizes ranging from 56 to 1,024 bits in
length. Data encryption is a complex subject far beyond the scope of this book. See
the references at the end of the chapter for resources providing information on data
encryption.