User`s guide

Chapter Six: Security Network address translation
69
• Only one computer on the internal LAN can be designated to receive the traffic for a specific protocol.
This means, for example, you can cannot create multiple web servers or FTP servers.
Default NAT server
When the DIVA LAN ISDN Modem receives incoming datagrams containing protocols it does not handle, it
discards them. For example, if an incoming datagram contains an FTP request and no FTP server has been
defined using a static mapping, the data is discarded.
In some cases, you may want to forward these datagrams to a specific computer. The DIVA LAN ISDN
Modem calls this computer, the default NAT server.
To define a default NAT server, enter the IP address of the device that should receive these datagrams into the
Default NAT server parameter on the IP Parameters panel (see page 70).
Note The DIVA LAN ISDN Modem handles traffic with the following protocols: HTTP, Telnet, TFTP, ECHO
(UDP port 7), and SNMP. Only traffic that does not contain these protocols will be forwarded to the default
NAT server. To forward these protocols you must define a NAT static mapping for each one.
Running a web or FTP server
Two of the most common reasons to use NAT static mappings are to run a web server or FTP server on a
computer connected to the DIVA LAN ISDN Modem.
Use the following diagram as a reference as you examine the NAT mapping examples that follow:
Configuring a NAT static mapping for a web server
Web traffic is carried on TCP port 80. Therefore, the following NAT mapping is required:
To access the web server, a remote computer uses the URL: HTTP://223.132.212.39/
The DIVA LAN ISDN Modem forwards all web traffic to the appropriate computer on the internal LAN.
Note
Setting up a web server disables remote access (via the ISDN line) to the DIVA LAN ISDN Modem web
interface. Local computers can still log in.
Configuring a NAT mapping for an FTP server
FTP traffic is carried on TCP ports 20 and 21. Therefore, the following two NAT mappings are required:
B-channel
DIVA LAN ISDN Modem
Internet
ISP
FAULT PO WE R
DIVA LAN ISDN Modem
ISDN
E4/CE1 E2 E3
LINK
Ethernet
B1 B2 D
192.168.1.1
192.168.1.5 192.168.1.4 192.168.1.3
192.168.1.2
223.132.212.39
Static IP address
FTP Server Web Server
Network
ISDN