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Ports not open on Arris CM8200

Level 2

Arris CM8200 cable modem - Property of Telstra?

Archer VR1600v - TPG wireless router


After my home broadband installation, access to all external ports are blocked. This restricts functionality on my network.


I can see that the ports are open on my LAN Archer VR1600v router, using Nmap, however, via an external scan, the ports are closed. I believe that this can only be result of blocking ports on the cable modem or due to dynamic IP.


Apparently Telstra disable all port on the Arris CM8200 cable modem, by default? 

I am unable to check the configuration of the cable modem, as it appears the standard IP address of has been reconfigured.


Although my external IP address is not static, this should not affect the scan or access to ports??


When I had ADSL2, I was able to configure these settings on the router and had no cable modem sitting between the modem and the internet. Although I did have a static IP address.


How do you suggest I regain control of my service, so that I have access to open ports on my external wan connection?



Level 15

If you think the standard IP address has been changed, you can check with your Windows computer. Open a command window and enter the command: ipconfig.

This shows your computer's ip address and the default gateway ip address (the modem).

If you then can't connect to the modem's web server, Telstra may have disabled it.

The CM8200 manual doesn't mention anything about blocking ports.


Is there anyone who can scan your system from the internet?


You could use an ethernet traffic analyser to capture ethernet traffic into your computer to see what happens to connection attempts to these ports you have open. You can see if they are getting through the CM8200.   (Freeware program called Wireshark.)