DHCP assigned unregistered IP address

Bob Hall rjhjr at cox.net
Thu Jul 14 17:56:34 GMT 2005

Thanks for all the replies. Responses to everyone's comments and

1) It is a Motorola cable modem. (SB5100)

2) I can access 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 != (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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