IP Subnet Calculator

What is IP subnetting?

Every device on a network has an IPv4 address, and subnetting is how that address space is split into smaller networks. A CIDR notation like 192.168.1.10/24 pairs an address with a prefix that says how many of its bits are "network" bits (shared by every device in the subnet) versus "host" bits (unique to each device). Subnetting lets you carve one large address block into smaller, separately-routable networks — the same math behind home router "IP ranges" and cloud VPC subnets.

How to use IP Subnet Calculator

1. Enter an address in IP address (or click Example for a sample).
2. Enter a prefix, mask, or plain number in Prefix or mask — e.g. /24, 24, or 255.255.255.0.
3. Read the network address, broadcast address, usable host range and subnet mask under Results.
4. Click Copy to copy the full result.

How the subnet math works

An IPv4 address is a 32-bit number, usually written as four decimal octets. A CIDR prefix (/24) says how many of those 32 bits are fixed as the "network" portion; the remaining bits identify individual hosts inside that network. The network address zeroes out every host bit, and the broadcast address sets every host bit to 1:

192.168.1.10/24
→ Mask:       255.255.255.0   (24 network bits, 8 host bits)
→ Network:    192.168.1.0
→ Broadcast:  192.168.1.255
→ Usable:     192.168.1.1 – 192.168.1.254  (254 hosts)

The usable host count follows RFC 4632's formula 2^(32 - prefix) - 2, subtracting the network and broadcast addresses from the total block size. Two prefixes are special cases per RFC 3021: a /31 has exactly 2 usable addresses (no broadcast — both addresses in the block are valid hosts, meant for point-to-point links), and a /32 identifies exactly 1 host with no network or broadcast concept at all. This tool builds the address as a real 32-bit integer and runs the mask math with bitwise operators — no string-based octet arithmetic, which is where off-by-one errors on the /31 and /32 edge cases usually creep in.

Frequently asked questions

What does CIDR notation mean?

CIDR (Classless Inter-Domain Routing, RFC 4632) writes a subnet as an IP address followed by a slash and a prefix length, e.g. 192.168.1.0/24. The prefix length is how many leading bits of the 32-bit address are the network portion; the rest are host bits.

Why does a /31 subnet have no broadcast address?

RFC 3021 defines /31 as a special case for point-to-point links: both addresses in the 2-address block are usable as host addresses, since a link with only two devices doesn't need a separate broadcast address. A /32 similarly has no network or broadcast portion at all — it identifies exactly one host.

How is the number of usable hosts calculated?

For a /0 through /30 prefix, usable hosts = 2^(32-prefix) - 2, subtracting the network and broadcast addresses. /31 has exactly 2 usable addresses and /32 has exactly 1, as special cases.

Can I enter a dotted subnet mask instead of a CIDR prefix?

Yes. The prefix field accepts /24, 24, or 255.255.255.0 — all three are parsed and normalized to the same result.