DHCP Scope
Enter the subnet, where the gateway sits and how many addresses you reserve or exclude, and get the usable DHCP pool, the projected client demand and whether it fits (the 80 % rule). Pure subnet arithmetic, nothing is sent to any server.
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DHCP pool usage
Rule of thumb: keep usage under 80 % for lease headroom.
How the subnet is split
| Item | Addresses | Note |
|---|
How it is calculated · pool and demand
1. Total addresses = 2^(32 − prefix).
Usable = total − 2 (network and broadcast removed). Special cases:
/31 = 2 (RFC 3021) and /32 = 1.
2. DHCP pool = usable − gateway (1 if applicable) − excluded − reservations.
3. Demand = clients × (1 + growth/100),
rounded up.
4. Usage = demand / pool × 100. Verdict:
sufficient if the pool covers demand with headroom,
tight if usage exceeds 80 % and insufficient if demand
exceeds the pool.
Runs locally in your browser · no sign-up · nothing leaves your browser
How it works
The calculator takes a subnet in CIDR notation (for example 192.168.1.0/24) and works out how many addresses are actually available for the DHCP pool: it starts from the usable ones (2^(32 − prefix) − 2, with the special cases /31 = 2 per RFC 3021 and /32 = 1) and subtracts the gateway (if it takes an address), the addresses excluded for statically configured devices, and DHCP reservations by MAC, which also consume pool.
With the pool resolved, it compares against the projected demand: demand = clients × (1 + growth/100), rounded up. Pool usage is demand / pool × 100 and the verdict applies the practical 80 % rule: sufficient when there is headroom, tight when usage exceeds 80 % and insufficient when demand exceeds the pool. Keeping margin prevents lease exhaustion with guests and new devices.
Example: office with 192.168.1.0/24 and 120 clients
- Usable:
2^(32−24) − 2 = 254addresses. - DHCP pool:
254 − 1 (gateway) − 5 (excluded) − 10 (reservations) = 238addresses. - Demand with 20 % growth:
120 × 1.2 = 144projected clients. - Usage:
144 / 238 ≈ 60 %→ sufficient, under the 80 % rule.