3com 3C886 Manual De Usuario

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. 

Figure 8   Connection to an Remote Private Network via an ISP

OfficeConnect 56K LAN Modem

192.168.1.1

Analog POTS line

Internet/Intranet or

online service

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

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

Internet Service

Provider

LAN Modem

Tunnel

terminator

Corporate

network