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First And Last Usable IP Calculator

Work out first and last usable ip instantly with clear inputs, formula shown and shareable results.

About First And Last Usable IP Calculator

Once the network address is known, the first assignable host is one above it and the last is one below the broadcast address. For a /31 and /32 there is no broadcast to skip, so the whole block is assignable. This is the range you hand to a DHCP scope or a static addressing sheet.

Formula

Usable range

first = network + 1; last = broadcast - 1 (for prefix <= 30)

network
IP AND mask
broadcast
network + 2^(32-prefix) - 1

Worked example

With the default values below, First And Last Usable IP Calculator returns First usable IP of 10.20.30.129. Change any input above and every figure here is recalculated in your browser.

Inputs used

IPv4 address
10.20.30.200
Prefix length (/n)accepts 1–32 bits
25 bits

Results

First usable IP
10.20.30.129
Last usable IP
10.20.30.254
Usable addresses
126
CIDR block
10.20.30.128/25

Step by step

  1. 1Values used
  2. 2Usable range
  3. 3First usable IP= 10.20.30.129
  4. 4Last usable IP= 10.20.30.254
  5. 5Usable addresses= 126
  6. 6CIDR block= 10.20.30.128/25

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Should the gateway come from this range?

Yes. The default gateway must be an assignable host address, and by convention it is the first or last usable address in the subnet.

Why is a /32 shown as a single address?

A /32 has no host bits, so it describes exactly one interface — commonly a loopback or a host route — and there is no network or broadcast address to exclude.

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