CIDR Subnet Calculator — IT Ops Tools

CIDR Block

Subnet Planner

Enter a network below, then click Divide on any block to split it in half — or click a Join cell to merge blocks back into a supernet. Divide different blocks to different depths to build a variable-length (VLSM) plan. Everything runs in your browser; the permalink saves your plan.

Network address

Mask bits

Mask accepts prefix bits (e.g. 16) or a dotted mask (e.g. 255.255.0.0).

Show columns: Subnet address Netmask Range of addresses Useable IPs Hosts Divide Join

Examples:

Save this plan for later — permalink to this subnetting.

How the CIDR Subnet Calculator works

Six client-side calculations covering subnetting, splitting, and route summarisation — nothing you type leaves your browser.

Network & Broadcast Boundaries

Enter any CIDR block or bare IP to instantly get the network address, broadcast address, usable host range, subnet mask, wildcard mask, and total/usable host counts.

Visual Subnet Planning (VLSM)

Divide any block into two with a single click, and keep dividing individual blocks to different depths to build a variable-length plan — or split the whole network evenly to a chosen prefix. Ideal for VLANs, site allocations, and right-sizing address space.

Binary Visualisation

See exactly which bits make up the network portion versus the host portion of an address, with the network/host boundary clearly marked — built for teaching and for sanity-checking masks by eye.

Join / Supernetting

The planner's nested Join column merges adjacent blocks back into a larger supernet with one click — collapse a pair of buddies or an entire branch, so you can summarise routes, ACLs, and firewall rules without recomputing masks by hand.

RFC 1918 & Special-Use Awareness

Recognises private (RFC 1918), loopback, link-local, multicast, and other reserved ranges so you can spot whether a block is routable on the public internet at a glance.

Runs Entirely in Your Browser

All math happens client-side in JavaScript — nothing you type is sent to a server, logged, or stored. Safe to use against real internal address plans.

Frequently asked questions

How many hosts are in a /24?

A /24 network contains 256 IP addresses, of which 254 are usable for hosts — the first address is the network address and the last is the broadcast address.

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation writes an IP address followed by a slash and the number of network bits, for example 192.168.1.0/24. The prefix length says how many leading bits identify the network; the remaining bits are available for hosts.

What is the difference between a subnet mask and a wildcard mask?

A subnet mask marks the network bits as 1s, such as 255.255.255.0 for a /24. A wildcard mask is its inverse, 0.0.0.255, and is used in ACLs and OSPF to indicate which bits are allowed to vary.

How do I split a network into smaller subnets?

Open the Subnet Planner tab, enter your network, and click Divide on any block to split it in half. Divide different blocks to different depths to build a variable-length (VLSM) plan, or use Split evenly to carve the whole network into equal blocks. Click a Join cell to merge blocks back into a larger supernet.

Is this subnet calculator free and private?

Yes. Every calculation runs entirely in your browser in JavaScript — nothing you enter is sent to a server, logged, or stored, so it is safe to use with real internal address plans.