[unilynx]

To keep things manageable and prevent any broadcasting node from reaching every system you have, you generally slice your address space into smaller networks

Each network needs at least one gateway inside that network to reach the rest of the world, so that IP address is lost. Conventionally you also lose the lowest and highest IP address in a block to the network number and broadcast address (you can free those up, but it might confuse devices or admins)

So for every split or subdivision you make, you lose at least 3 addresses.

Conventionally you also split a network at a specific bit boundary (CIDR) so each network has 2^n IP addresses in it. If you went for n=4, you have 16 addresses and already lost 3 to the above factors, so now you have 13 addresses left.

If you happened to have 14 machines attached to that network, you need a network with 2^5 = 32 IP addresses, and you've got 15 unused IP addresses with no way to give them back to a higher level.

All quite similar to why a phone area code may not be able to make use of all phone numbers inside that area, but have no way real practical way to give them to other networks.