@David-TP 

Hi, when I first set up Deco I've set DHCP range from .20 to .250, enrolled all my devices to Deco Wifi, made DHCP reservations, and then changed DHCP range from .220 - .250. Why I did it like that? Two reasons

- I don't want DHCP range to be that big (.20 - .250)

- a lot of mobile devices need address reservation in my home but also they have to be able to connect to other WiFi networks outside my home using DHCP (kids are not tech savvy so I don't want them to change IP settings on their mobile devices)

This setup above works and no need to change that. I have 3 PCs which have static IP addresses defined in their network properties. Other devices are TVs, IoT,  surveillance cameras, mobile phones, tablets, smartwatches...

"DHCP server shows 25 IP addresses assigned" can be seen in Deco mobile app under "DHCP Server" option. It's shown on the top above "Start IP". New devices that sometimes fail to connect are different and are not model-dependent because when I increase DHCP range they can connect. Some of those devices are mobile phones from my guests for example. 

Regards.

  @David-TP 

Bumping post as reminder :D

  @PetarV IP addresses are leased by the DHCP server for a given duration (decided by the DHCP server).  I believe the Deco will count as "assigned" an address until its lease expires.  If an address is leased to a device that does not remain online, the address does not become availaible, the device owns the address until the lease expires.

  @PetarV 

" I don't want DHCP range to be that big (.20 - .250)": it is a wish, not a reason.

"a lot of mobile devices need address reservation in my home but also they have to be able to connect to other WiFi networks outside my home using DHCP": that is a good reason to reserve addresses, not to reduce the range of DHCP addresses.

Why do you not reserve addresses within your range of DHCP addresses?  The Deco will, obviously, not assign a reserved address to a device not having the MAC address configured for the reservation.
 

  @PetarV Assuming your DHCP lease duration is one day, you can not have more than 31 DHCP devices per period of 24 hours.

New devices would be unable to connect until some DHCP lease expires.

  @yves_b 

I don't have any issues with reserved addresses (all my devices have reserved IP addresses except guest devices). All devices which are reserved work.

My initial question was "how does deco count used DHCP addresses?" 

DHCP range has 31 addresses, from that range I have maybe 1 device that has an address assigned and not reserved (that should leave me with 30 free)

I have 25 devices with reserved IP addresses outside DHCP range and that works, no need to change that 

Deco on the other hand reports 28 addresses used. If I count all my devices currently online (18) I don't have that number, not today not in the last 2,3 days. 

I love my Deco and since I moved from range extenders to it I was reborn. Don't get me wrong I'm not criticizing it I need an explanation for DHCP option. Being +25 years in IT industry I do know something about networking (hence why I don't use range with +100 addresses in a home environment).

  @PetarV 

You are incorrectly assuming that all used addresses are assigned to online devices.

Once an address is leased to a device, the address remains "used" for the duration of the lease.

  @yves_b 

I know how DCHP works lol! As I wrote, in past 2,3 days there was exactly ONE device connected to my WiFi which doesn't have reserved IP address, yet Deco reports 28 addresses assigned. Where did this number 28 came from, is Deco counting reserved addresses even they are out of DHCP range in my case? End of question. If it does again number is wrong. How long lease is, 1d, 2d a week?

  @PetarV 

Some devices, such as Windows, show the duration of the lease they obtained.

The out of range addresses were leased by the Deco, hence I expect they will be counted in the amount of assigned addresses.

I observe that the count of assigned addresses does not decrease when the lease expires.