Subnet Calculator

The Full Subnet Picture

Given an IPv4 address and a CIDR or netmask, the tool computes network address, broadcast address, first and last usable host, total usable hosts, total addresses, wildcard mask (inverse of the subnet mask, used in Cisco ACLs), the class, whether it's private (RFC 1918) or loopback, and the binary representation of every relevant value. One page, the whole picture — no need to piece together four different calculators.

CIDR Basics

CIDR (Classless Inter-Domain Routing) notation expresses a subnet as 'network/prefix' — 192.168.1.0/24 means the first 24 bits (192.168.1) are the network portion and the last 8 bits are the host portion. A /24 gives you 256 addresses total with 254 usable for hosts (the network address and broadcast are reserved). Smaller prefixes (like /16 or /8) give you more addresses; larger prefixes (like /28 or /30) give you fewer. CIDR replaced the old Class A/B/C system because it allowed arbitrary allocation sizes rather than fixed power-of-256 blocks.

Edge Cases: /31 And /32

A /32 refers to a single host — common in routing tables for 'this specific address' routes. A /31 (two addresses) was historically invalid because both addresses would be consumed by network and broadcast, leaving no usable hosts. RFC 3021 redefined /31 for point-to-point links where the saved address space matters: both addresses are usable, no broadcast is reserved. The calculator handles both cases correctly: /32 counts 1 usable host, /31 counts 2.

Subnet Splitting

If you have a /24 and want to split it into smaller subnets for different VLANs or services, enter the longer prefix — splitting /24 into /26 gives four subnets of 64 addresses each. The tool lists each resulting subnet with its network, broadcast, and usable-host count. Max 1024 subnets displayed (going /8 to /24 would produce 65,536 which would be unhelpful in a scrolling table).

Binary Representations

For understanding how masks work (and for exam preparation), the binary views show the IP address, mask, network, and broadcast in dotted-octet binary. The network portion of the IP ANDs with the mask's 1-bits; the host portion is whatever's left. Once you see it in binary, the point of CIDR notation becomes obvious — /24 is just 'twenty-four 1-bits followed by eight 0-bits' in the mask.

What inputs does it accept?

Three formats: CIDR notation (192.168.1.0/24), IP with dotted-decimal mask separated by a space (10.0.0.0 255.255.255.0), or a bare IP which defaults to /24. Invalid masks (i.e. non-contiguous 1-bits like 255.0.255.0) are rejected.

Why are /31 and /32 different from a normal subnet?

Normally the network address and broadcast address are reserved, so usable hosts = total − 2. A /32 contains exactly one address, so it's used for pinning a single host (e.g. loopback) and we count that one as usable. A /31 under RFC 3021 is used for point-to-point links; it has two addresses and both are treated as usable hosts with no broadcast reservation.

Can I split a subnet into smaller ones?

Yes. Enter the new (longer) prefix — e.g. splitting a /24 into /26 subnets gives you four of them. The tool shows each resulting subnet's network, broadcast, and usable-host count. Max 1024 subnets shown to keep the table manageable.