Boston Linux & UNIX was originally founded in 1994 as part of The Boston Computer Society. We meet on the third Wednesday of each month at the Massachusetts Institute of Technology, in Building E51.

BLU Discuss list archive

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Discuss] Changing Comcast Modem to Bridged

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 

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

BLU is a member of BostonUserGroups
BLU is a member of BostonUserGroups
We also thank MIT for the use of their facilities.

Valid HTML 4.01! Valid CSS!

Boston Linux & Unix /