Wireless-N ADSL2+ Modem Router DGN2000 Reference Manual
Protecting Your Network 3-9
v1.0, July 2008
Considerations for Inbound Rules
• If your external IP address is assigned dynamically by your ISP, the IP address might change
periodically as the DHCP lease expires. Consider using the Dynamic DNS screen so that
external users can always find your network.
• If the IP address of the local server computer is assigned by DHCP, it might change when the
computer is rebooted. To avoid this, use the Reserved IP address feature in the LAN IP Setup
screen to keep the computer’s IP address constant.
• Local computers must access the local server using the computer’s local LAN address
(192.168.0.11 in the example in the previous figure). Attempts by local computers to access
the server using the external WAN IP address will fail.
Outbound Rules (Service Blocking)
The modem router allows you to block the use of certain Internet services by computers on your
network. This is called service blocking or port filtering. You can define an outbound rule to block
Internet access from a local computer based on the following:
• IP address of the local computer (source address)
• IP address of the Internet site being contacted (destination address)
•Time of day
• Type of service being requested (service port number)
Following is an application example of outbound rules.
Outbound Rule Example: Blocking Instant Messenger
If you want to block Instant Messenger usage by employees during working hours, you can create
an outbound rule to block that application from any internal IP address to any external address
according to the schedule that you create in the Schedule screen.You can specify that the modem
router logs any attempt to use Instant Messenger during this blocked period. You can also open or
close AOL or MSN Instant Messenger ports: see the Firewall Rules screen in the “Order of
Precedence for Rules” section on page 3-11.
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