DHCP assigned unregistered IP address

[Sean Hafeez]

Most cable modems have an RCF1918 on their external management  
address. This is not part of the routing you see. The modem bridges  
your NIC to the Cable headend interface - like a layer 2 ethernet. Do  
a TCPDUMP on your external interface. Here in Cox Cable land you will  
see packets from 10.x.x.x every once in a while. This is the traffic  
from you modem to the headend for management.




On Jul 14, 2005, at 10:56 AM, Bob Hall wrote:

> Thanks for all the replies. Responses to everyone's comments and
> suggestions:
>
> 1) It is a Motorola cable modem. (SB5100)
>
> 2) I can access 192.168.100.1 after modifying the firewall rules that
> stop RFC1918 nets on the outside interface. (IPFW) There's no checkbox
> to disable DHCP. I downloaded the modem user manual from the URL Glenn
> Dawson posted, but I didn't find anything on this problem.
>
> 3) I did the (shutdown, disconnect modem 30 seconds, reboot) cycle  
> about
> three times, but it didn't do anything. The solution was
>     # dhclient -r
>     # dhclient rl0
> Repeat until oip != 192.168.100.11. (I think it took four iterations,
> but I wasn't counting.) This cleared the IP address on both the inner
> and outer NICs. Maybe I should have specified the interface with  
> the -r
> flag, but it was easy to fix.
>
> 4) The problem was solved by the time I saw the dig suggestion, so I
> didn't have a chance to try it.
>
> 5) Bill was paid on time. :)
>
> The modem web page contained this:
>     The SURFboard cable modem can be used as a gateway to the
>     Internet by a maximum of 32 users on a Local Area Network (LAN).
>     When the Cable Modem is disconnected from the Internet, users on
>     the LAN can be dynamically assigned IP Addresses by the Cable
>     Modem DHCP Server.
> I take this to mean that the modem can't do NAT. It can only act as a
> gateway for registered IP addresses, which it can't assign.
>
> The config web page has two buttons:
>     Reset All Defaults
>     Restart Cable Modem
> I can't find any documentation on the second. Would "Restart Cable
> Modem" be equivalent to disconnecting the modem power for 30 seconds?
> And "Reset All Defaults" would have released the unregistered IP
> address? (Along with any other settings)
>
> How do I find the outward facing IP address for the cable modem? Is
> that the "option routers" line in dhclient.leases?
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