Julien Vehent on 11 Dec 2010 07:03:07 -0800

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Re: [PLUG] Comcast Cable modem & DHCP

On 12/11/2010 09:56 AM, Casey Bralla wrote:
On Saturday 11 December 2010 9:46:01 am Julien Vehent wrote:
I get an IP on the, and you seem to be on a private network, so I assume you commercial account works differently.

However, since is class A private network, non routable on
the internet, I'm wondering: what is your public ip address here ?

My public IP is, which is assigned by comcast to the modem via
dhcp.  The modem in turn then does NAT to a private subnet 10.1.10.x which I
connect to my Debian firewall box.  I then re-NAT this to 2 different private
subnets (192.168.2.x to my DMZ and 192.168.1.x to my internal network).

So your modem is a router. That's not my case, the modem acts as a bridge and the network card of the router behind (my linux box) receives the DHCP lease.

BTW, I did not buy a "static" IP form comcast, but I find the DHCP IP is
essentially static (years!) as long as you don't change the MAC of the

That's interesting, I'll monitor the leases to see if the IP changes over the next few weeks.

Or do you double NAT you packets ? Once on your linux box and then on
the comcast router again ?

On 12/11/2010 09:34 AM, Casey Bralla wrote:
I am doing something very similar on my comcast commercial account.  I
plug a Debian computer into one of the network ports on the comcast
modem and use this as a firewall and router to my DMZ and internal

I would like to replace the old linksys we are using in the house with a
Debian box connected to the modem (I want netfilter and traffic
control). The linksys is getting it's IP in DHCP mode already. From
what I see, the lease is 3 days long.

I thought a simple "dhclient eth1", the interface where the cable modem
is plugged in, would do the job, but it seems that it's refusing to
give my Debian a lease...


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