On 2022-03-20 12:50, Tim via users wrote:
On Sun, 2022-03-20 at 12:13 -0400, R. G. Newbury wrote:
'Configuring the DHCP server to work that way', is to set it to
deliver a static address. With a dhcp server, the problem is that any
change in the network, or the items connecting to it, can cause the
dhcp server to deliver a different address to a unit, while a static
address, once set as a static address, will not change. Moreover, a
static address setting is tied to the MAC of the unit, not its FQDN.
Again, I'd say generally not (addresses changing willy nilly). Unless
you have one of those cutdown DHCP servers which only doles out a tiny
number of addresses and has no choice but to share 4 addresses amongst
5 devices.
When a device boots up and tries to connect to the network, the DHCP
server sees this, and checks if the device has a prior lease. If so,
it tries to give it the same one again. The device can say it'd like a
particular address, but the DHCP server is boss and *can* honour or
ignore that request.
When another device boots up and tries to join the network, the same
thing happens again. The DHCP server sees if it's previously served
that device, and if it has, it'll try to assign it the same address as
last time. If it's a new device, it'll try to serve it an address it
hasn't given out before to anything else.
There are lease time parameters which can configure how short and how
long leases last for (e.g. try to keep the lease reserved for a few
days if possible, try to avoid changing leases within a few hours), but
they tend to be applied to what to do when the server has run out of
spare addresses and will have to re-use an address.
My experience is that the same devices usually don't get different
addresses, and different devices don't usually take over an address
used by something else. Part of that equation is the device, as well,
it'll usually ask the DHCP server if it can have the same IP as last
time, even if the lease period is over.
Of course that's no guarantee. Your DHCP server (such as in a router)
might have short default times. ISPs often deliberately pick short
times, so they can tell customers to disconnect, wait 10 minutes, and
try again, when things stopped working (the idea being that they may
get a different route, next time, and the ISP won't bother trying to
fault find their own network, or it might auto-reset devices when
they're free). On my LAN, I've set deliberately very long lease times,
just to avoid these nightmares. Leases will be held, if possible, for
months.
In my network, devices are assigned DHCP IP address according to their
MAC address. My wireless access point uses mac filtering.
With DHCP being assigned via MAC address, then they get a static IP
address fixed to that device. This helps with scripting for some
things. Something to consider.
Robin
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