Vmg4380-B10A/vmg4325-B10A

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NAT Port Forwarding

NAT Introduction

What is NAT?

NAT (Network Address Translation-NAT RFC 1631) is the translation of an Internet Protocol address used within one network to a different IP address known within another network. One network is designated as the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and "unmaps" the global IP addresses on the incoming packets back into local IP addresses. The IP addresses for NAT can be either fixed or dynamically assigned by the ISP. In addition, you can designate servers, e.g., a Web server and a Telnet server, on your local network and make them accessible to the outside world. If you do not define any servers, NAT offers the additional benefit of firewall protection. In such case, all incoming connections to your network will be filtered out by the CPE, thus preventing intruders from probing your network.

For more information on IP address translation, please refer to RFC 1631, The IP Network Address Translator (NAT).

Port numbers for some services:

Service	Port Number
FTP	21
Telnet	23
SMTP	25
DNS (Domain Name Server)	53
www-http (Web)	80

Environment

The NAT provides system administrators an easy solution to create a private IP network for security and IP management. Powered by NAT technology, the VMG4380-B10A/VMG4325-B10A supports complete NAT mapping and most popular Internet multimedia applications. This feature is the best described with the NAT port forwarding feature implemented in the CPE. In the case of the above diagram, we have an FTP server installed behind the CPE with an IP assigned by the local DHCP server (192.168.1.33). How should we configure the VMG4380-B10A/VMG4325-B10A, so that the notebook at the WAN site can access the FTP server? The following step-by-step procedure covers all necessary instructions.

Port Forwarding Configuration

Show the device information.

Click Status.

We can see that the WAN was assigned the IP = 111.251.185.173.

Create a port forwarding rule for the FTP server.

Go to Network Settings> NAT > Port Forwarding.
Select the Service Name, e.g. “FTP”.
Select the WAN Interface, e.g. “Data”.
Enter the External port Start, e.g. “21”.
Enter the External port End, e.g. “21”.
Enter the Internal port Start, e.g. “999”.
Enter the Internal port End, e.g. “999”.
Enter the Server IP Address, e.g. “192.168.1.33”.
Select the Protocol, e.g. “TCP”.
Click Apply.

A warning message as followed will pop up:

This phenomenon is normal, because the CPE itself can be accessed by the FTP, which also uses port 21. Since we are creating a new rule using port 21, the default port number of the CPE’s FTP server port will be automatically moved to 2121.

A new port forwarding rule is now created.

$c:\users\admini~1\appdata\local\temp\snaghtml14f41fec.png$
Show the IP configuration of the notebook:

$d:\document\2011\p-873hnup-51b\pic\19.jpg$
Use the notebook to access the FTP server with IP = 111.251.185.173.

$d:\document\2011\p-873hnup-51b\pic\20.jpg$

DMZ Host Configuration

If we enable the DMZ host, it will open up all the internal ports to the dedicated Server IP (in this case, IP = 192.168.1.35) allowing client at the WAN side to access the FTP server via port forwarding.

Create a DMZ host.

Go to Network Settings > NAT > DMZ.
Enter the IP of the Default Server Address, e.g. “192.168.1.35”.
Click Apply.

Use the notebook to access the FTP server with IP = 111.251.185.173.

$d:\document\2011\p-873hnup-51b\pic\20.jpg$

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