Re: [PLUG] Comcast Cable modem & DHCP

[Julien Vehent <julien@linuxwall.info>]

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 68.0.0.0/22, and you seem to be on a private
> > 10.1.10.0/24 network, so I assume you commercial account works differently.
> >
> > However, since 10.0.0.0/8 is class A private network, non routable on
> > the internet, I'm wondering: what is your public ip address here ?
>
> My public IP is 71.224.6.96, 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
> firewall.

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
> > > network.
> > > > 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...
> > > >
> > > > Thanks,
> > > > Julien

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