DHCP assigned unregistered IP address

[Bob Hall]

Tonight, during a thunderstorm, I shutdown the household FBSD internet
gateway. After the storm passed, I booted it up again, only to discover
that during the bootup, my ISP's DHCP server had assigned me a new IP
address. That wasn't a problem in itself, but the new address was
192.168.100.11, an unregistered address.

The result was that I could ping my ISP's DHCP server, but I couldn't do
anything that required a URL. When I talked to a tech at my ISP, she
couldn't see my modem. She insisted that the IP address wasn't a
problem because I had a router. (Huh?) After she scheduled a visit from
a tech for tomorrow and I hung up, I released the lease on the
unregistered address a few times until the DHCP server gave me a new,
registered address. Magically, everthing worked again. When I cancel the
tech visit tomorrow, I'll have them check to see if they can see my
modem again. Obviously, the unregistered IP address screwed up my system
and theirs.

My understanding, after extensive reading, is that the unregistered
address could only have come from my ISP's DHCP server. However, I don't
know why they would have assigned an IP address that screwed up their
systems and mine. I'm wondering if dhclient could have assigned this
address to itself if it encountered a problem during bootup. I don't
believe this could happen, but I don't understand why my ISP would shoot
itself (and me) in the foot.

Does anyone know enough about dhclient to say definitely whether it
could have been the culprit?  Anybody with enough experience with ISPs
to say why they would have assigned an unregistered IP address that
broke both systems?