Equipment (Modems,Gateways)
Modems, Gateways, and Networking Devices
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ksg
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NAT slow or fails (DPC3939B)

On our Comcast Cisco DPC3939B modem, we have several public IPs mapped 1-1 to LAN IPs for in-house servers.  This worked well for over a year, until the last couple of weeks, when people connecting in from the internet started getting dropped or very slow connections (in one test, a 33 kb file took over 12 minutes to transfer, a rate of about 0.0004 Mbit/sec). Connecting to the servers from the LAN still works fine, connecting to the internet from the LAN is fine, and bandwidth tests like Ookla are fine.

 

We have not changed anything on the modem or the LAN for months, which leads me to suspect some kind of hardware failure on the modem.  We have power-cycled the modem and servers, with no effect.

 

Is there some way to test NAT on the modem?  Cusadmin only seems to be able to test general connectivity.  Is it worth resetting and reconfiguring the modem?  Any other suggestions before I call support?  Thanks!

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ksg
Visitor

Re: NAT slow or fails (DPC3939B)

More info:  Inspecting traffic from the modem shows that about 98% of requests are coming through un-NAT-ted: they still have the internet address as their destination when they make it onto the LAN.

 

Contacted Comcast support, they tested the modem and confirmed that something was wrong, then a little later closed the ticket without fixing the NAT.

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ksg
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Re: NAT slow or fails (DPC3939B)

Another try with support, same result. It seems that the 1-1 NAT is gone for good.

 

The solution for us is to put the modem into a pass-thru mode (following the instructions by RCO in the post at http://forums.businesshelp.comcast.com/t5/Equipment-Modems-Gateways/How-to-enable-bridge-mode-on-DPC... (Thank you, RCO!)), and do the NAT and wireless (since wireless gets disabled) on devices sitting between the modem and the LAN.

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