IP subnet calculator

APP $ subnet ✓ There is no submit button. Address and subnet calculations do not require a server; they run in this browser PUB 2026-07-20UPD 2026-10-27VERIFY Standards and official sources

Select either a prefix or a subnet mask. The other updates automatically, and the calculator shows the IP address range. To split an existing IP range into smaller subnets, use ‘Subdivide’ below and see the results immediately. IPv4 only. The /31 results assume a point-to-point link. The calculator does not determine whether special-purpose addresses can actually be assigned.

INPUT

IP address (Address only — set the size by prefix or mask)

Prefix

Subnet mask

Results update as you type

RESULT

Using this tool

When it helps

Use this calculator to check an IPv4 address and mask before configuring a device, or to find the boundaries of a block in an address plan. Enter the IP address, then select either a prefix or a subnet mask; the other field updates to match. You can change the address to see whether it belongs to the same block, and distinguish the host range from the full range before using the values in a configuration.

Start with Block details for the current address. Compare subnets divides the parent block named on screen into equal-sized blocks, while Subdivide splits the current block itself into smaller subnets. These tabs use different starting blocks, so check both the address and prefix in the list heading. The Current marker identifies the block containing your input address.

Understanding results

Network is the first address in the block. Range includes every address from the first through the last, and Total addresses is the count of that complete range. For prefixes from /0 through /30, the calculator identifies the first and last addresses as the network and broadcast addresses, then excludes them from Usable addresses. This is a calculated host range, not a survey of addresses that are currently free.

Wildcard mask is the subnet mask with every bit inverted. In the binary display, compare the same bit positions in Input address and Mask to see which bits remain in Network. Even when your input is inside a block, Network shows the boundary address. Read the prefix and subnet mask from the values selected in the input fields above.

Measured case

In a browser calculation, entering 192.0.2.130 in the IP address field and selecting /26 produced the subnet mask 255.255.255.192. The result showed Network as 192.0.2.128, Broadcast as 192.0.2.191, and Total addresses as 64. The final number in the input address, 130, does not become the beginning of the block.

The same result showed 62 Usable addresses, from 192.0.2.129 through 192.0.2.190, and a Wildcard mask of 0.0.0.63. Comparing the total of 64 with the host count of 62 makes the exclusion of the two boundary addresses visible. The example uses an address reserved for documentation; the displayed count does not mean these addresses are available for assignment on a real network.

Questions and answers

Why does an address ending in /26 fail in the address field?

That field accepts only an IPv4 address. Set the size through Prefix or Subnet mask, so remove /26 from the address and select /26 in the appropriate field. Each part of the address must also be between 0 and 255.

Why are both addresses usable with /31?

The calculator applies the point-to-point link condition in RFC 3021. The two addresses identify the two endpoints; this does not establish that the same assignment works on an ordinary LAN. Under this condition, Broadcast is shown as Not applicable.

What does selecting /32 calculate?

A /32 identifies the single address you entered. The calculator shows one Total address and one Usable address, with Broadcast marked Not applicable. A single address cannot be divided into smaller blocks.

Why does the list show fewer rows than the subnet count?

Both subdivision tabs display at most 16 rows of blocks. If there are more, the total count and the range of displayed row numbers are stated separately, so do not count the visible rows as the complete set of subnets. This is a display limit.

Further details

How it operates

An IPv4 address has 32 bits. The prefix length tells you how many leading bits identify the network; the rest form the host portion. The calculator obtains the network address by applying a bitwise AND to the address and mask. Setting all host bits to one gives the last address in the range. The total address count is two raised to the number of host bits.

Extending the prefix by one bit divides a block into two blocks of half the size. Subdivide uses that relationship to calculate the number of blocks and the addresses in each. Its selector offers up to 12 longer prefix lengths beyond the current prefix. Calculating boundaries and deciding which addresses to assign to devices are separate tasks.

Standards and references

RFC 950 provides the address-mask and bitwise-AND basis for identifying subnets. The table in RFC 1878 is a reference for comparing prefixes, decimal masks, and total address counts. CIDR notation follows Section 3.1 of RFC 4632, while the condition for treating both /31 addresses as hosts comes from Section 2.1 of RFC 3021. These rules explain address calculations, not the assignment state of a real network.

Constraints

The calculator supports IPv4, not IPv6. It does not check special-purpose address restrictions, DHCP leases, addresses already in use, routes, or firewall settings, so the result cannot establish connectivity. JavaScript performs the calculation in your browser without connecting to the device at the entered address. Before applying the values, check the address plan for that network and the prefixes supported by its devices.

Standards: RFC 950 (subnetting procedure) · RFC 1878 (variable-length subnet table) · RFC 4632 (CIDR) · RFC 3021 (/31 point-to-point links)