IP Subnet Calculator

What CIDR math gives you

A subnet calculator takes an address and a prefix length in CIDR notation and works out the network address, the broadcast address, the usable host range, the subnet mask, and how many hosts the block holds. The prefix, the number after the slash, says how many leading bits are the network portion; the rest identify hosts within it.

When you reach for it

You use it when carving a VPC into subnets, when sizing a block for a given number of hosts, when writing firewall rules that reference a range, or when checking whether two addresses actually sit in the same network before debugging a routing problem.

Example

192.168.1.0/26
Mask:       255.255.255.192
Network:    192.168.1.0
Broadcast:  192.168.1.63
Hosts:      192.168.1.1 - 192.168.1.62  (62 usable)

A /26 leaves 6 host bits, so 2^6 = 64 total addresses. Two are reserved, which is where the next part trips people up.

Common pitfalls

The off-by-one that catches everyone is the broadcast and network reservation. A block of 64 addresses does not give you 64 usable hosts; the first address is the network identifier and the last is the broadcast address, leaving 62. Forgetting this makes a /30 point-to- point link look like it has 4 hosts when it only offers 2. The other trap is assuming addresses are in the same subnet because they look close: 192.168.1.62 and 192.168.1.65 are only three apart but a /26 boundary at .64 puts them in different networks.

FAQ

How many usable hosts does /24 give? 256 total minus network and broadcast, so 254 usable addresses.

Why can't I use the broadcast address for a host? It is reserved to address every host on the subnet at once, so no single interface may claim it.

What does /32 mean? A single host, zero spare bits. It is common in route tables and allowlists to pin one exact address, often one you found with IP info.