BYTETOOLS

DHCP Scope Calculator

Plan a DHCP scope from a subnet: pool start and end, exclusions, lease count, T1/T2 renewal timers and ready-to-paste dhcpd.conf and netsh config.

229
Pool size
254
Usable in subnet
12h
T1 renew
21h
T2 rebind

Scope plan

Network / broadcast
192.168.10.0 / 192.168.10.255
Subnet mask
255.255.255.0
Usable host range
192.168.10.1 – 192.168.10.254
DHCP pool
192.168.10.21 – 192.168.10.249
Default gateway
192.168.10.1
Simultaneous leases
229

Exclusion ranges

  • 192.168.10.1 – 192.168.10.20 (20 addresses — static reservations (gateway, switches, printers))
  • 192.168.10.250 – 192.168.10.254 (5 addresses — reserved at the top of the range)

Lease timers

Lease time
86,400 s (1d)
T1 — renewal (50%)
43,200 s (12h)
T2 — rebinding (87.5%)
75,600 s (21h)

At T1 the client unicasts a renewal to the server that gave it the lease; at T2 it broadcasts to any server. These are the RFC 2131 defaults — a server may send explicit option 58 and option 59 values instead.

ISC dhcpd.conf

subnet 192.168.10.0 netmask 255.255.255.0 {
  option subnet-mask 255.255.255.0;
  option broadcast-address 192.168.10.255;
  option routers 192.168.10.1;
  option domain-name-servers 1.1.1.1, 8.8.8.8;
  option domain-name "example.local";
  default-lease-time 86400;
  max-lease-time 172800;
  range 192.168.10.21 192.168.10.249;
}

Windows netsh commands

netsh dhcp server \\DHCPSRV add scope 192.168.10.0 255.255.255.0 "example.local LAN" "Generated by ByteTools"
netsh dhcp server \\DHCPSRV scope 192.168.10.0 add iprange 192.168.10.1 192.168.10.254
netsh dhcp server \\DHCPSRV scope 192.168.10.0 add excluderange 192.168.10.1 192.168.10.20
netsh dhcp server \\DHCPSRV scope 192.168.10.0 add excluderange 192.168.10.250 192.168.10.254
netsh dhcp server \\DHCPSRV scope 192.168.10.0 set optionvalue 3 IPADDRESS 192.168.10.1
netsh dhcp server \\DHCPSRV scope 192.168.10.0 set optionvalue 6 IPADDRESS 1.1.1.1 8.8.8.8
netsh dhcp server \\DHCPSRV scope 192.168.10.0 set optionvalue 15 STRING example.local
netsh dhcp server \\DHCPSRV scope 192.168.10.0 set state 1

Replace DHCPSRV with your DHCP server name. Windows adds the whole usable range and then excludes the static blocks, which is why the commands differ in shape from the ISC pool.

What is the DHCP Scope Calculator?

The DHCP Scope Calculator turns a subnet into a working scope plan. Give it the CIDR block, the gateway and how many addresses you want to hold back for static assignments, and it works out the usable host range, the exclusion ranges, the pool start and end, and how many simultaneous leases the pool supports.

  • Pool start, end and simultaneous lease count from any /8 to /30 subnet
  • Named exclusion ranges for static blocks at both ends of the subnet
  • RFC 2131 renewal timers — T1 at 50% and T2 at 87.5% of the lease
  • Ready-to-paste ISC dhcpd.conf subnet block with options 3, 6 and 15
  • Equivalent Windows netsh commands including exclusion ranges
  • Validates the gateway is inside the subnet and that the reservations leave a usable pool

How to use the DHCP Scope Calculator

  1. 1

    Enter the subnet in CIDR notation and the default gateway address.

  2. 2

    Set how many addresses to reserve at the start and end of the range for static devices.

  3. 3

    Choose the lease time and its unit — the T1 and T2 timers update as you type.

  4. 4

    Optionally add DNS servers and a domain name so they appear in the generated config.

  5. 5

    Copy the dhcpd.conf block or the netsh commands straight into your server.

About the DHCP Scope Calculator

The DHCP Scope Calculator turns a subnet into a working scope plan. Give it the CIDR block, the gateway and how many addresses you want to hold back for static assignments, and it works out the usable host range, the exclusion ranges, the pool start and end, and how many simultaneous leases the pool supports.

It also computes the renewal timers most people never think about: T1 at 50% of the lease time, when a client unicasts a renewal to the server that issued it, and T2 at 87.5%, when it starts broadcasting to any server. Both come straight from RFC 2131.

Finally it writes the configuration for you — an ISC dhcpd.conf subnet block and the equivalent Windows netsh commands, including the option numbers for the router, DNS servers and domain name. All of it is generated 100% locally in your browser; nothing about your network is uploaded.

Frequently asked questions

How do I calculate a DHCP scope range?

Start from the subnet's usable range — everything between the network and broadcast addresses — then subtract the addresses you want to keep for static assignments. For 192.168.10.0/24 with the first 20 reserved, the pool runs from 192.168.10.21 to 192.168.10.254.

What are T1 and T2 in DHCP?

T1 is when a client first tries to renew its lease, at 50% of the lease time, by unicasting to the server that gave it the address. T2 is at 87.5%, when it gives up on that server and broadcasts a rebinding request to any DHCP server that will answer.

How long should a DHCP lease be?

Eight to twenty-four hours suits a stable office network. Guest Wi-Fi and conference spaces want something much shorter — one to four hours — so addresses return to the pool quickly. Long leases reduce server traffic but strand addresses when devices leave.

How many devices can one DHCP scope serve?

The pool size, which is the usable addresses in the subnet minus your exclusions. A /24 with 20 addresses reserved at the start and 5 at the end supports 229 simultaneous leases. If you need more, either use a shorter prefix or add a second scope with a relay.

Should the gateway be inside the DHCP pool?

No — always exclude it. If the gateway address is inside the pool the server may hand it to a client, which causes a duplicate-address conflict and takes the whole segment down. That is why this calculator checks the gateway sits in the reserved block.

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