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Understanding VPNs
and PPTP
Jack's PC
192.168.1.2
Chrissy's PC
192.168.1.3
Larry's PC
192.168.1.4
Janet's PC
192.168.1.5
Figure 7 IP Address Translation
Virtual private networks (VPN) are private, secure networks created in public
networks such as the Internet. A VPN is essentially a secure, private tunnel within
the Internet. Since VPN calls are placed through a local ISP, they eliminate long
distance charges that would occur from directly dialing to a remote private
network.
One of the protocols which enables a VPN to be created is PPTP. The PPTP protocol
allows for multiple workstations to establish a secure multi-protocol connection to
a remote, private network via a single, locally-dialed ISP account as shown in
Figure 8. Any networking protocols such as IP, IPX and NetBEUI can be supported
transparently through the tunnel. While the LAN Modem supports PPTP, it does
not play an active role in creating or terminating a tunnel.
LAN STATUS
Alert
POWER
ISDN
B1
B2
TX COLL 1
2
3
4
OK
LAN Modem
Figure 8 Connection to an Remote Private Network via an ISP
Translates PC IP
addresses to IP
IP address
address assigned
assigned by ISP
by ISP
198.6.1.1
LAN STATUS
3
Com
3C886
PWR AA
CD
RD SD
OH
Tx Coll
1
2
3
4
Alert
56K LAN Modem
OfficeConnect 56K LAN Modem
192.168.1.1
Analog POTS line
Internet Service
3
Com
ISDN LAN Modem
Provider
3C892
Understanding VPNs and PPTP
Internet/Intranet or
online service
Analog POTS line
Public telephone
network
Tunnel
terminator
23
Corporate
network
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