What does CIDR notation like /24 actually mean?

The number after the slash is the prefix length — how many leading bits of the 32-bit IPv4 address are fixed as the network portion. /24 means the first 24 bits (the first three octets) identify the network, leaving the last 8 bits for host addresses within that network. A smaller prefix (like /16) means a larger network with more hosts; a larger prefix (like /30) means a smaller network with fewer hosts.

How do I calculate the number of usable hosts from a prefix?

Total addresses in the block is 2^(32 − prefix). Usable host addresses are normally that total minus 2, because the first address is reserved as the network address and the last is reserved as the broadcast address. For example, /24 gives 2^8 = 256 total addresses and 254 usable hosts. Two special cases: /31 (RFC 3021) has no separate network/broadcast address and both of its 2 addresses are usable for a point-to-point link, and /32 is a single host address with exactly 1 usable address.

What are the network and broadcast addresses, and why aren't they usable hosts?

The network address is the lowest address in a subnet (all host bits set to 0) and identifies the subnet itself rather than any device on it. The broadcast address is the highest address (all host bits set to 1) and is used to send a packet to every device on that subnet at once. Neither can be assigned to an individual device, which is why they're excluded from the usable host count.

What are the private IP ranges defined by RFC 1918?

RFC 1918 reserves three blocks for private networks that are never routed on the public internet: 10.0.0.0/8 (10.0.0.0–10.255.255.255), 172.16.0.0/12 (172.16.0.0–172.31.255.255), and 192.168.0.0/16 (192.168.0.0–192.168.255.255). This tool flags whether the address you entered falls inside one of those ranges.

Does this tool look up or send my IP address anywhere?

No. This is pure bit arithmetic on the address and prefix you type in — there's no DNS lookup, no WHOIS query, and no network request of any kind. Everything runs locally in your browser with plain JavaScript, so nothing you enter ever leaves your device.

IP SUBNET / CIDR CALCULATOR

Calculate network and broadcast addresses, subnet mask, wildcard mask, and usable host range from CIDR notation. Pure local arithmetic — nothing is looked up or sent anywhere.

CIDR (IP/prefix)

CIDR summary192.168.1.0/24 (private (RFC 1918))

Network address	192.168.1.0
Broadcast address	192.168.1.255
Subnet mask	255.255.255.0
Wildcard mask	0.0.0.255
Total addresses	256
Usable hosts	254
First usable host	192.168.1.1
Last usable host	192.168.1.254
IP class	C

How to use the subnet calculator

1
Enter an address in CIDR notation, like 192.168.1.0/24, or switch to IP + Subnet mask mode and enter both separately.
2
The tool instantly computes the network address, broadcast address, subnet mask, wildcard mask, and usable host range.
3
Copy the network address or the CIDR summary with the copy buttons, or read any other value straight from the results table.

Questions

What does CIDR notation like /24 actually mean?: The number after the slash is the prefix length — how many leading bits of the 32-bit IPv4 address are fixed as the network portion. /24 means the first 24 bits (the first three octets) identify the network, leaving the last 8 bits for host addresses within that network. A smaller prefix (like /16) means a larger network with more hosts; a larger prefix (like /30) means a smaller network with fewer hosts.
How do I calculate the number of usable hosts from a prefix?: Total addresses in the block is 2^(32 − prefix). Usable host addresses are normally that total minus 2, because the first address is reserved as the network address and the last is reserved as the broadcast address. For example, /24 gives 2^8 = 256 total addresses and 254 usable hosts. Two special cases: /31 (RFC 3021) has no separate network/broadcast address and both of its 2 addresses are usable for a point-to-point link, and /32 is a single host address with exactly 1 usable address.
What are the network and broadcast addresses, and why aren't they usable hosts?: The network address is the lowest address in a subnet (all host bits set to 0) and identifies the subnet itself rather than any device on it. The broadcast address is the highest address (all host bits set to 1) and is used to send a packet to every device on that subnet at once. Neither can be assigned to an individual device, which is why they're excluded from the usable host count.
What are the private IP ranges defined by RFC 1918?: RFC 1918 reserves three blocks for private networks that are never routed on the public internet: 10.0.0.0/8 (10.0.0.0–10.255.255.255), 172.16.0.0/12 (172.16.0.0–172.31.255.255), and 192.168.0.0/16 (192.168.0.0–192.168.255.255). This tool flags whether the address you entered falls inside one of those ranges.
Does this tool look up or send my IP address anywhere?: No. This is pure bit arithmetic on the address and prefix you type in — there's no DNS lookup, no WHOIS query, and no network request of any kind. Everything runs locally in your browser with plain JavaScript, so nothing you enter ever leaves your device.