[NCLUG] PPP and IP addresses

[Charles Clarke]

http://www.ietf.org/rfc/rfc2131.txt is the DHCP RFC.

If you are renewing a lease, you don't specify your address, the
server knows it.  If rebooting, you do request the old address.
You can use the old until it expires or you get a decline from the
server in response to a request.

Unless configured specially by the admin, most DHCP servers will renew the
same IP address as long as you want.

Most clients don't release the old address unless they know they are
moving between subnets. 

ATT Broadband (which I just cancelled due to them cancelling my new
account that I had gotten after the old one didn't work for 30 days and
most of the tech support being idiots - they wanted me to wait 1.5 weeks
for a new modem to be brought by a truck to replace my current modem
because "the modem is associated with the account")  would give an initial
lease of 1 hour and then when you renewed would give you a lease of 4
days.  Now, in my view, 4 days kinda defeats most of the benefits of DHCP. 
Assuming most folks would be on less than 8 hours a day, you could, at
least, double the # of machines that could be supported by a set # of IP
addresses by setting the lease time to 4 hours(which most would renew
after 2). 

charles

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On Fri, 11 Jan 2002, J. Paul Reed wrote:

> On Fri, 11 Jan 2002, Barbara Hayes wrote:
> 
> > >You know how DHCP leases you an address, and you have to renew it from
> > >time to time, and there is the possibility that you will be leased a
> > >different address?
> >
> > Yes, but not during the same session - I was told at the time I
> > investigated this that if you're on a lan, a "session" is the time
> > between rebooots.
> 
> That's not quite true... a DHCP client is supposed to reverify its settings
> based upon the lease time it gets each time it does so; if the lease is 5
> minutes, and the DHCP client and server are both following the specs (i.e.
> no Microsoft products), then, theoretically, your address could change
> every 5 minutes.
> 
> You can request a lease of any length you want, but the server isn't
> required to give it to you.
> 
> Similarly, you can request the same address, and most servers will give you
> the same address anyway (if your address was changing every few minutes,
> that would wreak havoc on TCP connections), but they're not required at all
> to do so.
> 
> If they were, that would make DHCP useless.
> 
> > Don't know if there's an upper limit on this.
> 
> Whatever the lease is... I think the lease is given in seconds, and so
> whatever the max value of that field in the DHCP config packet is.
> 
> Later,
> Paul
>     ------------------------------------------------------------------
>     J. Paul Reed            preed at sigkill.com || web.sigkill.com/preed
>     What's the point in being nuts if you can't have a little fun?
>                                    -- John Nash, Jr., A Beautiful Mind
> 
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