CIDR / Subnet Calculator
Enter a CIDR block like 10.0.0.0/16 and get the netmask, network address, broadcast address, first and last usable hosts and total address count. Built for network engineers, DevOps teams sizing VPC subnets, and anyone studying for networking certifications.
How to use
- Type a CIDR block such as 192.168.1.0/24 into the input.
- Press Calculate to expand it.
- Read off the netmask, wildcard mask, address range and usable host count.
CIDR (Classless Inter-Domain Routing) notation compresses a network definition into an address plus a prefix length: 192.168.1.0/24 means the first 24 bits identify the network and the remaining 8 bits address hosts. The arithmetic is simple but error-prone by hand, a /24 holds 256 addresses (254 usable after subtracting the network and broadcast addresses), a /16 holds 65,536, and every step down the prefix doubles the block.
The place most engineers meet this today is cloud networking. AWS VPCs accept blocks between /16 and /28, and AWS silently reserves 5 addresses in every subnet rather than the traditional 2, so a /28 gives you 11 usable IPs, not 14. Kubernetes pod CIDRs, WireGuard allowed-ips lists and firewall rules all use the same notation, and an off-by-one prefix (writing /8 when you meant /24) can accidentally whitelist 16 million addresses.
A few useful anchors: /32 is a single host, /31 is a two-address point-to-point link (RFC 3021), /24 is the classic 'class C' LAN, and 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are the RFC 1918 private ranges that should never appear on the public internet. The calculator also handles IPv6 prefixes, where a standard LAN allocation is a /64, 18 quintillion addresses per subnet, which is why nobody counts usable hosts in IPv6.
Examples
Input: 192.168.1.0/24
Output: Netmask 255.255.255.0, range 192.168.1.0-192.168.1.255, hosts 192.168.1.1-192.168.1.254, 254 usableInput: 10.0.0.0/16
Output: Netmask 255.255.0.0, range 10.0.0.0-10.0.255.255, 65,536 addresses (65,534 usable)Input: 203.0.113.16/28
Output: Netmask 255.255.255.240, range 203.0.113.16-203.0.113.31, 14 usable hostsFrequently asked questions
Why does a /24 have 254 usable hosts and not 256?
The first address is the network identifier and the last is the broadcast address, so both are reserved. 256 total minus those 2 leaves 254 assignable to hosts. Cloud providers such as AWS reserve additional addresses on top of this.
What does the number after the slash mean?
It is the prefix length: how many leading bits of the address are fixed as the network portion. A bigger number means a smaller network, /30 has 4 addresses while /8 has 16.7 million.
How do I know if an IP is inside a CIDR block?
Mask both the IP and the block's base address with the netmask. If the results match, the IP is inside. This calculator shows the full range so you can check by eye too.
Does this work for IPv6?
Yes. Enter a prefix like 2001:db8::/48 and you get the expanded range and subnet count. IPv6 has no broadcast address, so every address in the block is usable.
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