Different IP every time

Simon Hobson dhcp1 at thehobsons.co.uk
Tue Mar 6 15:59:56 UTC 2007

Dario Protulipac wrote:

>is it possible to force the DHCP server to give a client different IP
>address every time the lease expire? Something like the PPP LCP does.

Firstly, as far as I know, the PPP LCP does not specifically force a 
different address each time - you are probably just observing what 
specific implementations do.

As far as DHCP goes, the RFC requires the server to try and give each 
client the same address each time (not that Microsoft take any 
notice, their server ignores that along with several other 

What is your reason for wanting this ? I don't think we've ever had a 
legitimate reason, usually it's "we are an ISP and my manager insists 
we do this to stop <insert whatever>". Changing the clients IP 
address is bad, really bad, and very dumb.

If that is the case, read on ...

It will not stop clients running server type applications (that's why 
there are dynamic dns services). What it will do is break a clients 
connections at regular intervals. Lets say you pick two hours, and a 
client tries to download a big file that takes 2 1/2 hours to 
download. After 2 hours (or less) the download will break, and the 
user will probably try again, and again, and again. Many clients 
(particularly http downloads) don't resume broken downloads so each 
time the user is downloading <some large volume> of data which is bad 
for the internet, bad for the server, and bad for you as the ISP.

Eventually your customers will find out why their downloads don't 
work and other connections get broken, and you will get "bad vibes" 
that will make any worries about customers running servers look 
trivial. Customers will leave as fast as they can and you will go 
bust. Frankly, that is what stupid ISPs deserve !

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