$ ipcalc
Subnet / CIDR calculator for IPv4 and IPv6. Everything updates as you type, and the mask fields are linked: change any one and the rest follow. Click a result to copy it. Runs entirely in your browser.
Paste networks or addresses (one per line, or comma separated; IPv4 and IPv6 can be mixed). They are merged into the smallest list of CIDR routes that covers exactly the same addresses.
About the cidr tool
The subnet / CIDR calculator converts between prefix length, netmask and wildcard mask, and shows the network address, broadcast address, first and last usable host and number of hosts for IPv4 and IPv6. It also splits a network into smaller subnets and aggregates a list of routes into the fewest CIDR blocks.
How to read the results
A /24 has 256 addresses and 254 usable hosts (the network and broadcast addresses are reserved). Each extra prefix bit halves the block: /25 is 128 addresses, /26 is 64. /31 (point-to-point links) and /32 (single host) have special handling. For IPv6, /64 is the standard size for a LAN and /48 is a typical site allocation.
FAQ
- How many hosts are in a /24?
- 254 usable hosts out of 256 addresses.
- What is CIDR notation?
- It writes an address with a prefix length, e.g. 192.168.1.0/24, meaning the first 24 bits identify the network.
- How do I split a network into subnets?
- Choose a longer prefix. Splitting a /24 into /26 blocks gives four subnets of 64 addresses each.
- What is route aggregation?
- Combining adjacent networks into one larger prefix, such as two /24 blocks into a /23, to keep routing tables small.