BT SmartHub 2 keeps changing DMZ device IP

[WSH]

@licquorice Funnily enough, that's what I thought but I decided to wait and see what was said next.

[alman]

The device had its static assigned by the BT router,

But as the behaviour of that BT router was ..... challenging,

I started trying to move components away from the BT Router :

Setting up DNS and DHCP on another box, then tried a PiHole so assigned a static AND a DNS to it.

But.... still the BT box randomly applies different IP addresses to the DMZ, despite the device having the same....and never ever changing.

Sometimes it would give the correct device name, but some random ip, which is why I tried the DNS.

I've tried turning the DHCP back on with  a range above 200, 

the device still holds its low number ip, yet the DMZ assigned is random

[licquorice]

You can't assign a static address with a router, that is an oxymoron.

You need to give the device a true static address on the device itself.

[WSH]

Yes, static addresses by definition are NOT assigned by DHCP.  They are assigned by the administrator, manually on the device itself.  The only reference to DHCP is that they must be outside of the range being assigned by DHCP or DCHP will sooner or later issue the same address to another device, causing a conflict.

You may be confusing it with “reserved addresses” where they are assigned by DHCP but the same address is always assigned to the same MAC address.  The DMZ set up will be a lot easier with a proper static address.

[alman]

I meant Static DHCP lease, rather than static IP.

[WSH]

Well again, that is contradictory.  You can’t have a static lease.  By definition a ‘lease’ is not permanent.

[alman]

here’s how to setup static lease,

https://discourse.pi-hole.net/t/setting-up-dhcp-leases-for-static-ips/21514

Either way, I’ve now removed PiHole from the mix.

Ive set the devices up manually ( which has held through no problem there)

ive turned the dhcp of the bt router on with a high range , and disabled it.

same result , after an hour or so the DMZ ip is reset randomly.

[WSH]

I can see where you're getting the confusion from.

As I said before, a reserved address is not the same thing as a static address.  Reserved addresses can be flakey.  One place I worked wouldn't allow them for that reason.  (Then again, another place I worked did everything as reservations without any apparent problems).  I still stand by the comment that a proper static address will be more reliable.

[WSH]

As far as “static leases” go, what really happens is that the device will request a lease renewal before it expires and so a continually running device will appear to have a ‘permanent’ lease.  In fact, it is getting a new lease every few days.

In the case of a reservation, even if the lease expires, (because, say, the device was turned off), the device requests a new address and the address that it gets is the same one, with a new lease.  That’s the idea of a reservation.

Incidentally, just to clarify a reserved address must be within the DCHP scope.  A static one is outside of that scope.

[alman]

the ip on the device is static and configured within it.

the ip in the bt router for setting outside the DMZ changes in less than an hour to random numbers outside of the range set in the dhcp.

I’ve tried dhcp on and off with a small high range including the static ip and without.

the solution I’ve come up with is to turn the firewall off and to put another router on the inside , with the firewall turned on, where all my networking is attached to.

its a better router anyway than the bt one , I have more control. FritzBox that I had lying around