Ars Technica on IPv4 exhaustion

Laszlo Hanyecz laszlo at
Mon Jun 23 03:41:34 UTC 2014

On Jun 23, 2014, at 3:32 AM, "Kalnozols, Andris" <andris at> wrote:

> On 6/22/2014 7:41 PM, Frank Bulk wrote:
>> Did they ever explain why?  Did the SMC function as a router, and act as the
>> customer side of a stub network that allowed that /29 to hang off the
>> router?  If that was the case, and the Motorola D3 modem was L2-only, that
>> might explain the change in capability. 

The Comcast business SMC gateway speaks RIP to make the routed /29 work.. in theory it could be put into bridge mode and you can do the RIP yourself but they don't support that configuration (you'd need the key to configure it successfully and they didn't want to do when I asked).  If you poke around in the web UI, it does support IPv6 in some form, but it doesn't seem to be active for me.

If you don't have a static IP block from them and thus don't have the need to use RIP you can just use a regular DOCSIS 3 cable modem and get IPv6, but you only get one IPv4 number that way.


> They didn't really go into detail.  Your theory sounds correct; the
> four ports on the SMC router default to but will also
> handle a routable /29 address from the WAN side of another router
> plugged into it.
> Since Comcast now charges $19.95 instead of $9.95/month for a /29,
> I inquired about the cost of an IPv6 assignment; same price as I
> recall being told.  I then asked if that was for a /60 or /56 and
> he said no, eight IPv6 addresses (/125?).  I politely thanked him
> and ended the phone call.  I realize that I could have gotten a
> more realistic answer from another Comcast rep with more v6-fu
> but I didn't pursue it.
> Andris
>> -----Original Message-----
>> From: NANOG [mailto:nanog-bounces at] On Behalf Of Kalnozols, Andris
>> Sent: Sunday, June 22, 2014 9:29 PM
>> To: nanog at
>> Subject: Re: Ars Technica on IPv4 exhaustion
>> <snip>
>> My experience as a Comcast Business customer with a /29 IPv4 subnet was
>> that swapping out the SMC modem/router for an IPV6-capable Motorola
>> DOCSIS 3 modem meant that I could no longer have the /29.
>> Andris

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