What does the number after the slash mean in CIDR?

The number after the slash (e.g. /24) is the prefix length, it tells you how many bits of the 32-bit IP address represent the network. The remaining bits represent host addresses within that network.

How many IPs are in a /16 CIDR block?

A /16 block has 2^(32-16) = 65,536 total addresses, of which 65,534 are usable (excluding network and broadcast addresses).

What is a private IP range?

Private IP ranges are reserved for internal networks: 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B), and 192.168.0.0/16 (Class C). These addresses are not routable on the public internet.

What is supernetting?

Supernetting is the opposite of subnetting, it combines multiple smaller networks into a larger one by using a shorter prefix length. For example, 192.168.0.0/23 covers both 192.168.0.0/24 and 192.168.1.0/24.

CIDR notation calculator

Enter any CIDR notation to understand what it represents. The tool breaks down the network address, prefix length, subnet mask, and the complete range of IP addresses in the block.

CIDR block

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

/24 · 255.255.255.0 · 254 usable hosts

How it works

The tool parses the CIDR notation using pure JavaScript bit arithmetic. The prefix length determines the subnet mask; bitwise AND gives the network address; bitwise OR of the inverted mask gives the broadcast address. All host range calculations follow from there.

Processing runs in your browser

All calculations happen locally in the page. Our servers are not involved at any point.

Technical specification

Classless Inter-Domain Routing (CIDR) is defined in RFC 4632 (IETF, 2006). CIDR notation expresses an IPv4 network as address/prefix where the prefix length (0–32) specifies how many leading bits constitute the network portion. The subnet mask is derived by setting the top n bits to 1 and the remaining 32 − n bits to 0. Network address = address AND mask; broadcast = address OR (NOT mask). IPv6 CIDR follows the same principles with 128-bit addresses per RFC 4291.

Standard: RFC 4632. Classless Inter-domain Routing (CIDR)
Address space (IPv4): 32-bit; /24 = 256 addresses, /16 = 65,536
Subnet mask derivation: Top n bits = 1, remaining 32 − n bits = 0
Usable hosts: 2^(32−prefix) − 2 (network + broadcast reserved)

Related operations

For working with file permission bits, try the chmod calculator. To convert IP octets between binary and decimal, use the number base converter. For converting between KB, MB, and GB in transfer planning, see the byte converter.

Frequently asked questions

What does the number after the slash mean in CIDR?: The number after the slash (e.g. /24) is the prefix length, it tells you how many bits of the 32-bit IP address represent the network. The remaining bits represent host addresses within that network.
How many IPs are in a /16 CIDR block?: A /16 block has 2^(32-16) = 65,536 total addresses, of which 65,534 are usable (excluding network and broadcast addresses).
What is a private IP range?: Private IP ranges are reserved for internal networks: 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B), and 192.168.0.0/16 (Class C). These addresses are not routable on the public internet.
What is supernetting?: Supernetting is the opposite of subnetting, it combines multiple smaller networks into a larger one by using a shorter prefix length. For example, 192.168.0.0/23 covers both 192.168.0.0/24 and 192.168.1.0/24.