[Discuss] Changing Comcast Modem to Bridged

[jbk at kjkelra.com (jbk)]

On 12/30/18 11:55 AM, Robert Krawitz wrote:
> On Sun, 30 Dec 2018 11:39:24 -0500, jbk wrote:
>> On 12/30/18 11:01 AM, Robert Krawitz wrote:
>>> On Sun, 30 Dec 2018 10:39:06 -0500, 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.
>>> I've done that by the simple expedient of connecting our domestic
>>> router to our (RCN -- it doesn't matter) router, and having everything
>>> else except for my server system connect to our domestic router.  In
>>> other words, a DMZ topology.  The only thing I had to configure on the
>>> RCN router was the port forwarding to any ports I want open on the
>>> server.
>>>
>>> If you don't have a static IP/no open ports, it's even easier; the
>>> only two things on the DMZ are the ISP router and the domestic
>>> router.
>> The problem with doing that is there is no way to turn off the
>> Comcast dhcp server w/o putting it into bridged mode, other than
>> limiting the range to a single address and have that lease set to
>> forever. But forever on the modem is only as long as that device
>> remains connected without interruption. If there is an interruption,
>> power outage, then the first device detected on resumption will get
>> that address lease.
> You don't need to turn off the DHCP server on the Comcast router,
> because you're only going to have one device attached to it, your
> domestic router.  Everything else connects to your domestic router,
> which you configure as you please; you connect the domestic router's
> uplink to the Comcast router and let the Comcast router assign the
> domestic router whatever address it pleases.
>
>> Thanks Robert, I would have to do a lot more research to see what
>> setting up a DMZ would mean to my topology and the dhcp issue is the
>> biggest headache and bridge mode is the only way to turn it off on
>> the modem.
> Here's the configuration I'm suggesting.  The "=====" network is the
> DMZ, but it's simply an ethernet connection:
>
> 					+---- Internal device 1
> 					|
> INTERNET ----- Comcast ===== Domestic --+---- Internal device 2
> 					|
> 					+---- Internal device ...
>
> >From the standpoint of the Comcast router, it sees that it's connected
> to the Internet and to one device internally (you can even turn off
> the wireless altogether on it).  From the standpoint of the domestic
> router, it sees the Comcast router as the internet, and all of your
> other devices connected to it.
>
> Mine's a little different because of my server, which is connected to
> both the DMZ and the internal domestic network but doesn't route:
>
> 					+---- Internal deviceS
> 					|
> INTERNET -----ISP/dhcp1 ======    Router+dhcp2/WAP/VAP
> 		     			|
> 		     			+-----internal
> 		     			      Server

Currently all of my devises point to ISP/dhcp1's IP as the 
gateway.

Would I now want to have these devices point to the modem 
assigned dhcp address of Router/dhcp2 or the LAN address I 
assign to dhcp2. This is where my confusion is.


-- 
Jim Kelly-Rand
jbk at kjkelra.com