[Discuss] Changing Comcast Modem to Bridged

[jbk at kjkelra.com (jbk)]

On 12/30/18 10:49 AM, Dan Ritter wrote:
> jbk wrote:
>> A couple years ago we changed to comcast as our ISP and incorporated their
>> modem into our network topology providing the dhcp, NAT and wireless
>> functions.
>>
>> Prior to this we had a DSL modem and WRT54G running tomato. The modem
>> provided dhcp so it was the gateway address.
>>
>> I now want to put the Comcast modem in bridge mode and have my wireless
>> router running dd-wrt provide the dhcp and NAT for the wireless and wired
>> LAN.
>>
>> According to the research I've done there are only two ip address options
>> for setting up the modem in bridge mode. 10.0.0.1 or 192.168.100.1.
>>
>> My current network subnet mask is 255.0.0.0 for the dozen or so devices that
>> have static IP's. I do not provide any services outside the local LAN but
>> within I have a backup server that serves a number of devices.
>>
>> As I understand it the modem IP in bridged mode wants to be on a different
>> subnet that the internal LAN which would lead me to believe that the 192
>> prefixed IP address would be the choice, this is question #1
>>
>> Once I've setup the modem with the correct IP then will the router now
>> become the gateway?
>>
>> I have the Cisco DPC3941T modem, has anyone on here set up the bridge
>> themselves, I see the option in the management GUI, or per my web searching
>> this change can only be done correctly by the right Comcast personel
>> remotely?
>>
>> Well that's the gist of it, did I leave out anything?
> You've got some confusion in there.

Yes, I set this network up around 2001 with the help of 
someone else that new networking better than I.


>
> 1. NAT has to be handled by a router which has at least one
> outside address and at least one inside address.
/Which would now be my router in bridge mode./
> 2. DHCP can be done by any device on the inside.
>
> 3. A bridge operates at the ethernet level, not the IP level.
>     So once it's in operation, you pretend it's a chunk of wire:
>     your router connects to the bridge and uses the Comcast
>     assigned outside address(es), and connects to your internal
>     network with internal address(es).
/So the router LAN address will now become the gateway 
address on my internal network?/
> A subnet mask indicates how large a chunk of the IP space should
> be considered as local. You've got 16.7 million addresses
> considered local right now...
/Yes, I only learned this later after I had made all these 
other static decisions/
> The net says:
>
>   Cisco DPC3941T modem
>
> 1) Press and hold reset button on back of gateway for 30
> seconds, this will reset the gateway back to the factory
> defaults..
>   
> 2) Connect a computer to ethernet port #2 on the back of the
> gateway.
>   
> 3) After the gateway boots, verify computer has connectivity,
> connect to gateway @ 10.0.0.1
>   
> 4) Change the gateway's login password, disable both private
> wifi networks, set ipv4 and ipv6 firewall to custom mode and
> select option disable/none.
>   
> 5) Set Gateway > At a Glance > Bridge Mode to Enable. When you
> see the timer screen pop up, you can disconnect the computer
> from the gateway as it is rebooting. the reboot can take 3 - 5
> minutes.
>   
> 6) Connect your router to the gateway ethernet port #1, on the
> router make sure that the WAN / Internet link is set to disabled
> or off.
>   
> 7) Once the gateway completes it's boot cycle, enable the
> Internet WAN on the router. The router should now have the IP
> address issued by Comcast.
>   
> 8) Configure the router as you see fit.
>
> Hope that helps.
>
> -dsr-
>
>
Thanks Dan, I did see the instructions on the net of the 
steps you outline here, but out of context of my network. 
DD-WRT has a lot of helpful information on their site and 
forums and I am confident once the modem is in bridge mode I 
can implement a network more tailored to our needs including 
a VAP to get our frequent guests off our local LAN.

Jim

-- 
Jim Kelly-Rand
jbk at kjkelra.com