Cactus
Subnet calculator
Enter an IPv4 or IPv6 address with a prefix (CIDR) to see the network and broadcast addresses, the usable host range, the mask, and what kind of address it is.
How this works
- This is pure math — Cactus works it out from the address and prefix and never connects to anything.
- What you enter is used to show this result and isn't stored.
Frequently asked questions
What is CIDR notation (the "/24" part)?
The number after the slash is how many bits at the front of the address are fixed for the network - the rest are free for hosts. So /24 locks the first 24 bits (255.255.255.0) and leaves 8 bits, or 256 addresses, for the block.
How many usable hosts does a subnet have?
For IPv4 it's the total addresses minus two - one for the network address and one for the broadcast - so a /24 has 256 - 2 = 254. The exceptions are /31 (2 usable, a point-to-point link) and /32 (a single host). IPv6 doesn't reserve those two, so every address in the block is usable.
What's the difference between the network and broadcast address?
The network address is the first in the block (all host bits 0) and names the subnet itself; the broadcast address is the last (all host bits 1) and reaches every host at once. Neither is assigned to a device, which is why they don't count as usable hosts on IPv4.