Subnet Calculator

Enter an IPv4 network, then click any address block to divide it into two equal subnets. Join sibling blocks whenever you want to combine them again.

Base network

Host addresses are automatically aligned to the selected network boundary.

IPv4 address

Prefix length

CIDR

Network192.168.1.0/24

Netmask255.255.255.0

Addresses256

Current subnets1

Click to divide

Each block represents its share of the base network. Select a block to split it into two smaller CIDR ranges.

BaseDivided onceDivided twiceDivided three or more times

Subnet plan

Use Divide or Join for precise control. Host counts follow RFC 3021 for /31 networks.

Subnet	Netmask	Address range	Usable range	Hosts	Actions

How to use the visual subnet planner

Enter an IPv4 address and CIDR prefix, such as 192.168.1.0/24.
Select Load network. A host address such as 192.168.1.42 is aligned to 192.168.1.0 automatically.
Click a visual block—or its Divide button—to create two equal child networks.
Use Join on either child to recombine a sibling pair.

What subnet division changes

Each division adds one bit to the prefix. Dividing a /24 produces two /25 networks; dividing one of those produces two /26 networks. The address ranges never overlap and together continue to cover the original network.

Usable host addresses

For prefixes from /0 through /30, the first address is the network address and the last is the broadcast address. A /31 is treated as a two-address point-to-point network, while a /32 represents one host route.

Worked example

Dividing 192.168.1.0/24 creates 192.168.1.0/25 and 192.168.1.128/25. Each child contains 128 total addresses and 126 traditionally usable host addresses.