[Discuss] Changing Comcast Modem to Bridged

[dsr at randomstring.org (Dan Ritter)]

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.

1. NAT has to be handled by a router which has at least one
outside address and at least one inside address.

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).

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...


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-