Dwight Victor wrote:
...
If you keep your cable modem (or windows PC, or DHCP client process)
running 24/7, it will request that the DHCP server renew the lease on the
current IP address whenever the lease period expires. If your cable modem
(or windows PC, or DHCP client process) is off longer than the lease
period, then when you power back on (or restore, or reload, or whatever),
you will be assigned a new IP address. This is all standard DHCP stuff.
Check the RFCs for details.
Dwight...
Correction: You *might* be assigned a new address.
While I don't believe the RFC requires it, most DHCP servers (including the
extremely popular ISC DHCPd) will attempt to hold leases for you as long as
possible even after they expire. For example, I've had people come to my house
for a LAN party and then return months later and get the same IP address. Why?
I have 240 IP addresses in my DHCP pool and there have never been that many
computers on my network. The DHCP server kept giving out new addresses rather
than use one that had previously been given out. Even if more systems are on
the network than IPs available, most servers will use the longest expired
addresses first in an attempt to give frequent clients the same address.
This isn't to say this is the tried and true behavior. Some servers may just
give out addresses randomly (assuming they aren't in use, hopefully...). Some
ISPs (like cable modem providers) may even desire this since they want to sell
you static IPs at a premium. YMMV.
The surest way to keep your IP static (and this works pretty well) is to just
keep your system running. I have only had like 4 IP changes in 1.5 years since
getting my cable modem. 3 times were because the ISP changed the subnetting
(and thus had to let all the leases expire), and one was due to hardware failure
on my end resulting in my server/router being down so long as to be unable to
renew its IP address.
Just keeping your cable modem on will probably *not* hold your IP for you. Most
cable modems just act as bridges (they don't have an IP that matters). To hold
your IP, make sure that the system using that IP is able to renew the IP
regularly (at least before the lease expires). The client should be able to
tell you how long the lease is (here they are 24 hours). In addition, many
clients allow you to force a renew if you want to buy some more time.
If you would like to change your IP, you can try releasing and renewing. This
usually doesn't work. See above and MS DHCP servers seem to ignore DHCP
releases (their clients don't bother, so why should the server?).
--MonMotha
