[geeks] Airport Express arrived yesterday!

[Scott Howard]

On Sat, Jul 17, 2004 at 08:12:15PM +0100, Mike Meredith wrote:
> Some ISPs operate 'DHCP' servers out of spec to avoid customers thinking
> they own a particular IP address. Strictly speaking these aren't DHCP
> servers at all.

Pretty much wrong on both accounts.
Presuming you're refering to things like ADSL/cable DHCP servers, the
fact the IP changes on each connection is because most ADSl/cable
routers don't request the same IP address whenever they re-connect.
Most routers will issue a DHCPDISCOVER request on reconnect, rather than
a DHCPREQUEST/INIT-REBOOT request. With a DHCPDISCOVER the server is
free to give the client whatever address it wants - even if there is an
existing lease for the client. By default, most servers will attempt to
give the same address as the client previously had, but there's no
requirement for it to.

As far things being/not being "true" DHCP servers, the spec is very
loose, and very much written to give freedom to the DHCP server admins
to do whatever they want.

eg, the section of the RFC on renewing leases says :
" A client MAY choose to renew or extend its lease prior to T1. The
server MAY choose to extend the client's lease according to policy set
by the network administrator. The server SHOULD return T1 and T2, and
their values SHOULD be adjusted from their original values to take
account of the time remaining on the lease."

So even if the ADSl/cable router is trying to renew an existing lease
(and like I said, most of them dont) then the server isn't really under
any obligation to renew/re-certify the address, which will cause the
client to go into INIT state and request a new IP.

  Scott