How to Use This Tool
Set how many devices are present at peak, how many new ones arrive each day, and how long they stay. The lease time is the setting that decides everything else.
A lease outlives the device
When a device disconnects it does not, in general, release its address. It simply stops talking, and the server keeps the binding reserved until the lease expires. Anything that leaves without a clean release — a phone walking out of range, a laptop lid closing — leaves an address held.
So the number that matters is not how many devices are connected, it is how many have connected within one lease period. With 200 arrivals a day:
- 8 days — about 1,600 addresses held. A /24 has 254.
- 1 day — about 200.
- 8 hours — about 67.
- 2 hours — about 17.
Eight days is the default in most DHCP servers, and it is the right answer for an office where the same laptops return each morning. On a guest network it fills the pool with devices that left days ago, and the failure looks baffling: clients cannot get an address while the network appears almost empty.
Matching the lease to the network
- Office with assigned devices — days. Long leases mean a device keeps the same address across a reboot, which makes logs readable and firewall rules stable.
- Guest wifi — two to four hours. Long enough that a normal visit never renews, short enough that the pool recovers by the afternoon.
- Café or retail — 30 minutes to an hour. Visits are short and volume is high.
- Conference — short, and size for the burst rather than the average. Everyone arrives within the same twenty minutes.
What a short lease costs
Clients attempt renewal at 50% of the lease time, and again at 87.5% if the first attempt fails. So a two-hour lease means every device contacts the server hourly, where an eight-day lease means every four days.
For a few hundred devices this is nothing. It becomes worth considering at large scale, and it matters for battery on mobile devices, since a renewal wakes the radio. There is no benefit to a lease shorter than the pool actually needs.
Sizing the scope
Once you know the addresses held, add headroom and reserve space for anything static. The prefix choices are coarse, so the practical question is usually whether you have outgrown a /24:
- /24 — 254 usable
- /23 — 510
- /22 — 1,022
- /21 — 2,046
A single flat broadcast domain of more than about a thousand devices is usually a bad idea for other reasons — broadcast traffic, failure blast radius, wireless roaming. If the arithmetic says /21, the real answer is more likely several smaller subnets than one enormous one.
Things this does not model
- Devices with several interfaces. A laptop docked by cable and also on wifi takes two addresses.
- Randomised MAC addresses. Phones that rotate their address per network can take a new lease each time, which multiplies churn on guest networks in a way nothing warns you about.
- Failover. A pair of servers sharing a scope typically splits it, so each holds only part of the range and neither can allocate the whole pool alone.
