Understanding VPNs and PPTP 23

Figure 7 IP Address Translation

Understanding VPNs
and PPTP 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.

Figure 8 Connection to an Remote Private Network via an ISP

192.168.1.5
192.168.1.2
192.168.1.4
OfficeConnect 56K LAN Modem
192.168.1.1 Analog POTS line
Internet/Intranet or
online service
3 Com
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
198.6.1.1
198.6.1.1
198.6.1.1
198.6.1.1
IP address
assigned by ISP
Translates PC IP
addresses to IP
address assigned
by ISP
Analog POTS line
Public telephone
network
3C886
56K LAN Modem
PWRAA RDSD
Alert TxColl 1 2 3 4 CD OH
LAN STATUS
From the Edge to the Heart
of the Network.
3 Com
POWERISDN
Alert B1B2 TX COLL 1 23 4
3 Com
LAN STATUS
ISDN LAN Modem
3C892
OK
Internet Service
Provider
LAN Modem Tunnel
terminator Corporate
network