Access to Time Warner cable network

Matthew Seaman matthew at
Sun Apr 1 14:35:19 UTC 2012

On 01/04/2012 14:35, RW wrote:
> I had a modem that did something similar, it issued a temporary private
> ip address and the replaced it with a routable address.

It's fairly sad that they don't use the officially mandated[*] netblock which is what DHCP clients/servers are supposed
to use when they need to temporarily grab an address.

> The difference here is that the DHCP server is in a different address
> block to the DHCP server, but I'm not sure that's a problem. I think
> that FreeBSD associates  DHCP traffic with the interface its operating
> on irrespective of normal routing.

Huh?  One of those servers should be a client perhaps?

Yes.  Contacting a DHCP server is done using Ethernet protocols (at
least initially.[+])  Not using IP.  That means DHCP client and server
have to be on the same ethernet segment, or there should be a DHCP-relay
on any routers between the client and server.  If that fails, then the
client can assign itself a link-local address and try that, but it is
pretty uncommon in the wild.

While you can run multiple different IP networks over the same physical
ethernet segment, and so have DHCP servers that dish out addresses on
networks distinct from any they have configured on their own interfaces,
you're more likely to run into this sort of scenario if there are some
DHCP relays in the picture.



[*] RFC 5735

[+] Well, also except for IPv6 -- DHCP6 just uses the auto link-local
addresses which are pretty much always configured on any IPv6 capable

Dr Matthew J Seaman MA, D.Phil.

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