DHCP Scope Calculator: Pool Range, Size and Lease Time

Calculate a DHCP start/end range, count available addresses, and check current utilization. Estimate lease demand, choose a suitable subnet, and get lease-time guidance for a new or existing IPv4 scope.

All calculations run locally in your browser. Validate production changes against DHCP server logs, IPAM records, and network policy.

Inputs

Network address or CIDR Example: 192.168.10.0/24. A bare address defaults to /24; a host address with /prefix is normalized to its network.

Peak or active clients Use the highest observed active-lease count when available.

DHCP pool start First address in the configured distribution range.

DHCP pool end Last address in the range; the pool count is inclusive.

Lease duration (hours) 24 hours = 1 day. T1/T2 remain server-configurable.

Growth buffer (%) Applied after the larger of observed peak and modeled demand.

Advanced sizing

Network profile Sets an evidence-qualified starting range, not a universal rule.

Modeled steady clients Normally present at the design peak.

Transient arrivals per day Short-lived clients that can leave before leases expire.

Average visit duration (hours) Used to estimate stale-address pressure after a client leaves.

Exclusions inside pool Addresses in start-to-end range the server must not lease.

Reservations inside pool Count only reservations that consume this pool; do not repeat exclusions.

Target utilization (%) Configurable planning assumption; 80% is a cautious starting point.

Scope presets

Results

Enter the scope details, then calculate.

Scope status-

Assignable pool-

Planning utilization-

Recommended subnet-

Pool capacity

Detailed results

Subnet and pool

Subnet mask:-

Network address:-

Broadcast address:-

First usable address:-

Last usable address:-

Total / usable addresses:-

DHCP start – end:-

Inclusive pool size:-

Addresses outside pool:-

Exclusions / pool reservations:-

Usable leases after exclusions/reservations:-

Demand and warnings

Observed active utilization:-

Modeled stale-address pressure:-

Modeled demand:-

Planning basis (greater input):-

Projected leases with growth:-

Required assignable / gross pool:-

Safe capacity remaining:-

Warning trigger:None

Lease-time guidance

Suggested range:-

Current lease / T1 / T2:-

Normal T1 renewals per hour:-

Configuration drafts

Drafts include only the calculated network, mask, pool range, and lease duration. Review syntax, options, interfaces, gateways, DNS, failover, and policy before use.

Selected draft

How to use this DHCP scope calculator

Choose a mode: check a deployed start/end range or size a new scope.
Enter the network: supply an IPv4 network address or CIDR; existing-scope mode also needs the pool start and end.
Add the default demand inputs: peak or active clients, lease duration, and growth buffer.
Refine if needed: open Advanced sizing for exclusions, pool reservations, modeled clients, churn, target utilization, and network profile.
Review and implement carefully: use the recommendation, capacity segments, detailed arithmetic, lease guidance, and optional configuration draft.

DHCP scope concepts

Subnet: The address block defined by a network address and prefix or mask.
DHCP scope: The server's administrative address and option grouping for clients on a subnet.
Dynamic pool: The inclusive start-to-end range available for allocation; it does not have to cover the whole usable subnet.
Exclusion: An address inside the configured scope range that the DHCP server must not dynamically assign.
Reservation: An address associated with a particular client. It reduces general assignable capacity when it is inside the pool and not already counted as excluded.
Active lease: An address currently recorded as leased, which may include a client that has already left.
Utilization: Leases or projected demand divided by assignable pool capacity.

Annotated /24 example: a scope can be smaller than the subnet.

.0
network.1–.19
outside pool.20–.239
DHCP pool.240–.254
outside pool.255
broadcast

Formula and assumptions

Inclusive pool: end IP integer - start IP integer + 1

Assignable pool: inclusive pool - exclusions inside pool - reservations consuming pool capacity

Stale transient pressure: arrivals/day × max(lease hours - visit hours, 0) / 24

Modeled demand: steady clients + stale transient pressure

Planning basis: max(observed peak, modeled demand)

Projected leases: planning basis × (1 + growth buffer)

Required assignable pool: ceil(projected leases / target utilization)

The 80% target and profile lease ranges are configurable planning assumptions. IPv4 boundaries and inclusive range counts are deterministic arithmetic. T1 and T2 are shown at the RFC defaults of 50% and 87.5% only when a server does not provide other values.

Worked DHCP calculations

Office /24 that should expand to /23

A /24 has 2^8 = 256 total and 256 - 2 = 254 usable addresses. Suppose 30 pool addresses are exclusions or reservations and the larger of observed or modeled demand is 185 leases. Add 20% growth: 185 × 1.20 = 222 projected leases. At an 80% target, the assignable pool must be ceil(222 / 0.80) = 278. Restore the 30 unavailable pool addresses: 278 + 30 = 308 gross pool addresses. A /24 cannot fit 308 pool addresses, while a /23 has 510 usable addresses, so /23 is the smallest suitable common subnet.

Guest Wi-Fi where a shorter lease reduces stale holds

Assume 120 steady clients, 240 arrivals per day, and a 4-hour average visit. With a 24-hour lease, stale pressure is 240 × (24 - 4) / 24 = 200 addresses, for 320 modeled leases. With an 8-hour lease, it becomes 240 × (8 - 4) / 24 = 40, for 160 modeled leases. The shorter lease releases about 160 stale holds sooner, but it also produces more renewal traffic and must be tested against real client behavior.

IPv4 capacity and lease strategy tables

These are starting points, not universal standards. Pools can cover only part of the usable subnet.

Prefix	Mask	Total	Usable hosts	Example pool plan
/29	255.255.255.248	8	6	Up to 4 dynamic + 2 infrastructure
/28	255.255.255.240	16	14	10 dynamic + 4 infrastructure
/27	255.255.255.224	32	30	24 dynamic + 6 infrastructure
/26	255.255.255.192	64	62	50 dynamic + 12 infrastructure
/25	255.255.255.128	128	126	100 dynamic + 26 infrastructure
/24	255.255.255.0	256	254	200 dynamic + 54 infrastructure/growth
/23	255.255.254.0	512	510	400 dynamic + 110 infrastructure/growth

Network behavior	Starting lease range	Why
Stable (home, office, IoT)	1–7 days	Low churn; fewer renewals, but slower recovery of abandoned addresses.
Mixed (office Wi-Fi, lab)	8–24 hours	Balances turnover with renewal traffic.
High churn (guest, event)	1–12 hours	Recovers departed-client addresses sooner; validate client compatibility and traffic.

Standards, sources, and review

Standards-based arithmetic is kept separate from planning choices. Network/broadcast boundaries and inclusive IPv4 range counts are mathematical; T1/T2 defaults come from DHCPv4. Target utilization, churn retention, and lease-profile ranges are adjustable operational assumptions that should be checked against logs.

RFC 2131: Dynamic Host Configuration Protocol — DHCPv4 lease behavior and default T1/T2 fractions.
Microsoft Learn: DHCP scopes in Windows Server — scope, exclusion, and reservation terminology.
ISC Kea Administrator Reference: DHCPv4 server — subnet, pool, and valid-lifetime configuration.

Methodology author: Starlight Robotics infrastructure tooling team. Technical review: protocol behavior checked against RFC 2131 and vendor documentation. Reviewed: 18 July 2026.

FAQs

What is a DHCP scope?

A DHCP scope is the administratively defined set of IPv4 addresses and options a DHCP server can use for clients on a subnet. Its dynamic pool may cover only part of the subnet's usable range.

How do I calculate a DHCP start and end range?

Find the network and broadcast addresses from the CIDR, choose a start and end inside the usable host range, and count the pool inclusively as end minus start plus one. Then subtract only exclusions and capacity-consuming reservations inside that pool.

How many DHCP addresses are in a /24?

A /24 contains 256 total IPv4 addresses and normally 254 usable host addresses. The DHCP pool can be smaller because it may use only part of that usable range and can contain exclusions or reservations.

Do reservations reduce pool size?

A reservation inside the configured dynamic pool consumes one pool address when that reserved client holds it. A reservation or static address outside the pool does not reduce that pool, and an address already counted as excluded must not be subtracted again.

What causes DHCP scope exhaustion?

Common causes include too small a pool, growth, device churn combined with long leases, abandoned leases, excessive exclusions or reservations, unexpected clients, and overlapping or incorrect scope configuration.

What lease time should guest Wi-Fi use?

Four to twelve hours is a useful starting range for many guest networks, but the correct value depends on visit duration, daily churn, renewal traffic, and authentication policy. Validate it with lease history and client behavior.

Can two DHCP scopes overlap?

Two independently allocating scopes must not offer the same address range. Deliberate failover or split-scope designs require coordinated, non-conflicting allocation rules; accidental overlap can produce duplicate-address conflicts.

What are DHCP T1 and T2 renewal times?

Common DHCPv4 defaults are T1 at 50 percent of lease time and T2 at 87.5 percent of lease time unless the server provides different timers.

Does this calculator support DHCPv6?

No. This tool is for IPv4 DHCP scope planning. IPv6 address assignment usually uses different assumptions and much larger address spaces.

Is this calculator private?

Yes. Inputs are processed locally in your browser and are not sent to a server.

Disclaimer

This is an infrastructure planning aid. Validate DHCP scope changes with IPAM ownership, routing, firewall rules, relay configuration, DHCP failover design, and operational change-management requirements before applying them in production.