Subnet & CIDR Calculator
Work out the network and broadcast address, usable host range, netmask, wildcard mask and host count for any IPv4 CIDR block — then split it into smaller subnets. Everything is computed locally in your browser.
New to this? Read the subnet calculator guide →
All calculations run locally in your browser — nothing is uploaded.
How to use the subnet calculator
- Enter an IPv4 address with a CIDR prefix, e.g. 10.0.0.0/16. Any host address in the block works — 10.0.5.7/16 resolves to the same network.
- Read off the network and broadcast addresses, the usable host range, the netmask and wildcard mask, and how many hosts the block holds.
- To carve the block into smaller subnets, enter a longer prefix under Split and press the button — the first subnets are listed with their network addresses.
What the numbers mean
The prefix (the number after the slash) is how many leading bits are fixed as the network; the rest identify hosts. A /24 fixes 24 bits, leaving 8 for hosts: 256 addresses, 254 usable once you set aside the network and broadcast addresses. Each step down doubles the hosts: /23 is 512, /22 is 1024, and so on. The wildcard mask is the inverse of the netmask and is what ACLs and OSPF use.
Private vs public ranges
The tool flags RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), loopback (127.0.0.0/8) and link-local (169.254.0.0/16). Private addresses are reusable inside any network and are not routable on the public internet.
Frequently asked questions
What does a CIDR prefix like /24 mean?
The number after the slash is how many leading bits of the address are fixed as the network. A /24 fixes 24 bits and leaves 8 for hosts — 256 addresses, 254 of them usable. Each extra bit of prefix halves the block; each bit fewer doubles it.
Why are there fewer usable hosts than total addresses?
Two addresses in each block are reserved: the network address (all host bits 0) and the broadcast address (all host bits 1). So a /24 has 256 total addresses but 254 usable. A /31 is a special case used for point-to-point links where both addresses are usable.
How do I split a network into smaller subnets?
Enter a longer prefix in the Split box. Dividing a /24 into /26 subnets, for example, gives four blocks of 64 addresses each. The calculator lists the network address of each resulting subnet.
Is my IP data sent anywhere?
No. Every result is plain integer arithmetic computed in your browser, so you can plan private, internal networks without disclosing anything.