How to use the Subnet / CIDR Calculator
- Enter an IPv4 address with prefix, e.g. 10.0.0.5/20.
- Read network, mask, broadcast and host range.
- Open the cheat sheet for quick sizing.
About this tool
CIDR notation (192.168.1.0/24) describes a block of IP addresses: the number after the slash is how many leading bits identify the network. The remaining bits identify hosts. Cloud VPCs, firewall rules, Kubernetes pod ranges and office networks are all planned in CIDR blocks.
Two addresses in each normal subnet are reserved — the network address and the broadcast address — so a /24 has 256 addresses but 254 usable hosts. Point-to-point /31 links and single-host /32 routes are special cases, handled correctly here.
The private ranges defined in RFC 1918 — 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 — are flagged automatically. Our subnetting guide walks through the maths.
Frequently asked questions
How many hosts in a /26?
64 addresses, 62 usable.
What's a wildcard mask?
The inverse of the subnet mask, used in Cisco ACLs and OSPF.
IPv6 support?
Coming soon — IPv6 subnets are usually /64.