Subnet Calculator (IPv4 CIDR)

What does this subnet calculator do?

This tool takes an IPv4 address together with a CIDR prefix (for example 192.168.1.10/24) and works out every detail of the network block: the subnet mask, wildcard mask, network address, broadcast address, first and last usable host, the total number of addresses and the number of assignable hosts. Everything is computed in your browser with JavaScript — no IP is ever sent to a server, so it is safe to use even with sensitive internal ranges.

How to read the results

CIDR and subnet mask

The CIDR prefix tells you how many leading bits of the address belong to the network. If subnetting is new to you, our guide on what a subnet is walks through the fundamentals, and the subnetting cheat sheet lists every prefix at a glance. A /24 means 24 network bits, leaving 8 bits for hosts → a mask of 255.255.255.0 and 256 addresses. Each bit you add to the prefix halves the number of hosts: a /25 gives 128 addresses, a /26 gives 64, and so on.

Network, broadcast and the host range

The network address is the lowest address in the block (all host bits set to 0) and is not assigned to a device. The broadcast address is the highest (all host bits set to 1) and is used to reach every host in the subnet at once. The addresses in between make up the usable host range you assign to servers, routers and devices.

Special cases: /31 and /32

Per RFC 3021, a /31 network is used for point-to-point links and uses both addresses as hosts (there is no separate broadcast). A /32 describes a single host — handy when writing a firewall rule or a route for exactly one machine. To see the full first-and-last address span of any block, use the CIDR to IP range converter.

Real-world examples

• Split an office /24 into four /26 blocks for separate VLANs, each with 62 hosts.
• Check whether an address falls inside the allowed range of an AWS security group before opening a port.
• Quickly confirm that 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are private ranges (RFC 1918) when designing an internal network — see how IP addresses work for the difference between public and private space.
• Planning IPv6 instead? Switch to the IPv6 subnet calculator for prefix and address-count math on 128-bit networks.

Why calculating subnets by hand is error-prone

The most common mistake is getting the subnet boundary wrong when the prefix is not on an octet boundary (/26, /27…). Setting the wrong network address can leave a whole range unroutable, or worse, overlap with another subnet and cause IP conflicts that are painful to track down. Double-checking with a calculator before you push config to a router, a firewall or a Terraform file is always cheaper than debugging a network outage at 2 a.m.