Equipment (Modems,Gateways)
Modems, Gateways, and Networking Devices
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Visitor

NAT Loopback

Just switched over to business class today. I previously had xfinity service running. I am running A webserver on network behind a netgear router setup with ddwrt software. Before I switched i was able to access all webservers perfectly from within the network and externaly. Now with the new routher DPC3941B, it appears that i am having NAT loopback issues. From external devices i can access the webserver just fine, but internal not so much. The best i can do at this point is direct to the host ip but then i only get basic html, I have come to the conclusion that is because a lot of the info is useing ?discrete? links eg. "http://website.com" rather then /pics.

 

What i would like to do would be to get it working... but really. How can i setup the DPC3941B to act like the old router i had before. I just want to pass data though to the router. and let the router handle the rest.

 

Sorry im not a networking guys so please forgive me if this is stupid.

 

I have one static IP address

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Visitor

Re: NAT Loopback

So i have done some more testing.

 

If i type in my web address something.com with port forwarding turned on. external devices can access but nothing internal. But oddly if i turn port forwarding off, obiously nothing from the out side comes in but on an internal device it is actualy giving me my comcast modem interface.

 

 

I have found a temperary work around, by adding to my desktop computer's hosts file. doing this i am able to access the webservers with no problems. I realisticaly cant do this to every device on my network and would also rather not setup a localdns server.

 

 Does this confirm that i am haveing NAT loopback isues that are being caused by the modem itself?

 

And anyone have a good solution?

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Official Employee

Re: NAT Loopback

Hi Tallguy64 and welcome to the business forums. 

 

I can help with what you're looking for in the gateway. You're most likely looking to set up Passthrough Mode. 

 

Passthrough Mode describes a gateway which has its LAN DHCP and Firewall manually disabled through the user interface. This allows traffic to passthrough the modem to a routing device and can be used if the customer has a Static IP address. This is not true Bridge Mode and does not convert the gateway to a basic cable modem. This mode allows most of the routing capabilities of the gateway to remain intact. 

 

Please let me know how this works for you. 

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Re: NAT Loopback

Could you elaborate on how to set this up.

 

Disableing the fire wall is simple enough, and taking the DHCP offline i unederstand but from there what do i need to change?

 

Also by taking the lan dhcp offline would that mean that i would lose the wireless functionality in the comcast unit? I had planed on using the seperate wireless network for the home automation software i use to seperate that from my other computers and server. Not a deal breaker as i can setup my main router to have a guest wifi so that it is on its own network.

 

As of right now I setup a local dns server and that has seemed to fix the problem for now. My concern is that i might run into problems as the next thing i am setting up is the mail server and i am wondering if i will run into problems with the webserver doing loop back to the email server, but we will see.

 

Thank you for the kind answer, i have been crash coarsing myself though network administration LOL. and i am starting to understand how it is working, but still not completely there yet.

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Official Employee

Re: NAT Loopback

The LAN DCHP can be found under Connection > Local IP Network.

 

You're also correct with the wireless functions. Once disabling the DHCP, you would lose your wireless capabilities. Using your own router is definitely recommended. I am able to assist with configuring your gateway. However, please do keep in mind that I am not able to help set up your private network as this would be beyond the Comcast Business demarcation. Please let me know if you need anything additional. 

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Visitor

Re: NAT Loopback

Ok thanks, i think i should be fine for now but we will see.

 

I thinking i understand how to set the gateway that should be easy enough. I was just wondering with taking down the DHCP i would then set the router to a static IP, but would that IP be and internal ip 10.1.10.5 for example or would i set the router to have the ip of the static ip address?

 

Going out on a limb, if i were to get a second static ip so that i could run two seperate networks how would i set one ip to go to one router and one to go to the other? is this where the nat 1 to 1 comes into play?

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Official Employee

Re: NAT Loopback

Taking down the DHCP means you would have to manually assign the LAN IP to your device. I would recommend reaching out to a networking IT team for those questions. Please let me know if you need anything.

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