IPv4 Subnet Calculator

This calculator turns any IPv4 address and CIDR prefix into the numbers you actually need to configure a network. Type an address such as 192.168.10.0/24 and it works out the network address, broadcast address, the first and last usable host, the total and usable host counts, and both the subnet mask and its wildcard inverse.

It also shows the address in binary with a divider marking where the network bits end and the host bits begin, and it can split a block into equal smaller subnets, handy when you are carving a larger allocation into per-rack, per-VLAN, or per-service ranges.

What is subnetting?

Subnetting is the practice of dividing one IP network into several smaller networks. Instead of putting every device on a single flat range, you borrow bits from the host portion of the address to create additional network segments, each with its own address space.

Teams subnet for a few practical reasons:

• Routing: routers forward between subnets, so segmentation keeps broadcast traffic local and makes routing tables predictable.
• Security: putting databases, management interfaces, and public-facing services on separate subnets lets you enforce firewall policy between them.
• Efficient addressing: right-sizing each subnet to the number of hosts it needs avoids wasting large blocks on small segments.

Understanding CIDR notation

CIDR (Classless Inter-Domain Routing) notation writes a network as an address followed by a slash and a number, such as 10.0.0.0/24. The number is the prefix length: how many leading bits identify the network. Everything after those bits identifies hosts within it.

Because an IPv4 address is 32 bits, the smaller the prefix number, the larger the block. A /24 leaves 8 host bits and covers 256 addresses; a /27 leaves 5 host bits and covers 32; a /16 leaves 16 host bits and covers 65,536. Each step of one in the prefix halves or doubles the block size.

Subnet mask vs. wildcard mask

A subnet mask marks the network bits with 1s and the host bits with 0s. For a /24 that is 255.255.255.0. The first three octets are network, the last is host.

A wildcard mask is the bit-for-bit inverse: 0s where the mask has 1s, 1s where it has 0s. The /24 wildcard is 0.0.0.255. Wildcard masks show up in Cisco-style ACLs and in OSPF network statements, where a 0 bit means "must match" and a 1 bit means "don't care", so the calculator surfaces both so you can copy whichever your platform expects.

Reading the binary view

The binary view expands the address into its 32 bits, grouped into the four octets, with a divider placed at the prefix boundary. Everything to the left of the divider is the network portion and is fixed for every host in the subnet; everything to the right is the host portion and varies from address to address.

Reading it left to right makes the structure obvious: the all-zeros host portion is the network address, the all-ones host portion is the broadcast address, and every combination in between is a usable host. It is the clearest way to see why a /26, for example, splits a /24 into four blocks at the third-from-last host bit.