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New Contributor


5 Messages

Tue, Apr 5, 2016 10:00 AM

Cisco DPC3941B and port forwarding



It looks like I posted originally in the wrong subforum so perhaps posting in here can help me out.


We have a DPC3941B and tryign to setup port forwarding where an external port routes to a different internal port - for example port 5555 would route to port 80 internally on a specific server.


It appears the DPC3941B does not allow that - we can only setup a port range that is the same externally as it is internally. 


Aside from going bridge-mode and buying my own router - can the DPC3941B be modified to do proper port forwarding?






1.4K Messages

5 y ago

Hello g_pmw and welcome,


Not sure exactly what you mean by "proper port forwarding", but the industry standard enables ports to be opened on specific devices IP addresses. The DPC3941B within the Advance Mode, allows either static IP or LAN non-dynamic IP addresses to be open on any specific device. Now, if you open port 5555 and port 80, then your specific application running on that specific device must be able to handle the actual receive/transmit process routing. There is no real means by which you can actually force port 5555 to route (transmit/receive) data processing to port 80 via Port Forwarding. Again, as long as the ports are open on the specific device and the application routing is correct, then your data processing should be implemented successfully.


Could you give any other general information as to what is your objective and we could further attempt to provide additional assistance ?

New Contributor


5 Messages

5 y ago

Hello and thank you for the response!


I am trying to achieve exactly what I have outlined in my original post. While I understand the standard convention of port forwarding (i.e. external port equals internal port for a static lease or a regular static IP'd device), as you may be aware, there are plenty of devices (including Comcasts) that offer you to translate an external port to an internal port while specifying what port each one is - for example - port 55555 externally translating to port 80 internally for a specific device on the network.


Case in point - from Comcast's own guide for business IP gateways (see bold areas):


To add a new port forwarding rule:

  1. Select Add new. The Port Forwarding add/edit screen will display.

  2. In the Application Name field, enter an application name to identify this rule.

  3. Enter the port number range in the Public port field. The assignable ports are between 1 and 65535. The numbers should match whatever is required for the applicable service being forwarded (for example, http traffic will use port 80 by default). Users on the Internet will use the public port to connect to the LAN device for which you are creating this forwarding rule.

  4. Enter the first port of the port range in the Private port field. The assignable ports are between 1 and 65535. The private port is the port on the LAN PC, where this rule will forward traffic. Typically this will match the public range, but may differ in some cases. For example, SMTP traffic for email typically uses ports 25 and 110, but the mail server on the LAN can be configured to listen for requests on alternative ports. In these cases, the public ports will be set to the ports that the traffic type uses by default while the private port range will be set to match the needs of the server. The size of the private port range must match the size of the public port range and is automatically calculated for you.

  5. Select the appropriate protocol from the Protocol drop-down menu (TCP, UDP, or both).

  6. Enter the IP address of the device you want the traffic to be forwarded to in the IP Address field. If the destination device is connected to a router which connects to the Gateway, forward to the router IP, then create another forwarding rule in the router to the destination device. Select Connected Computers to locate the IP addresses of the devices connected to the Gateway.

  7. Select Apply. The new port forwarding rule will display in the port forwarding table.


99% of conventional routers offer the functionality described above as, apparently, Comcast leased routers as well. Unfortunately, the specific unit I have does not allow you to choose a different private port versus the public port and so I was hoping there is some unknown/hidden workaround that would allow me to do that aside from setting the Cisco gateway as a bridge and buying my own router.




19 Messages

4 y ago

Update about this router is it's on recall by Comcast. Just spent an hour mucking about trying to get it to do simple port forwarding.


Mispellings in the firmware and generally a horribly written firmware. How it passed QC is amazing.

Occasional Visitor


6 Messages

4 y ago

Wow, interesting.  I've heard nothing about this recall- mine is a POS too.  Anywhere you can point to that talks about this?  Thanks!