CIDR (Classless Inter-Domain Routing, pronounced "cider") is a method for allocating IP addresses and routing. It replaced the old "classful" system and allows for more flexible division of IP address space.
What Does CIDR Notation Look Like?
CIDR notation combines an IP address with a suffix indicating the network portion. For example:
- 192.168.1.0/24 - A network with 256 addresses (192.168.1.0 to 192.168.1.255)
- 10.0.0.0/8 - A large network with 16 million addresses
- 172.16.0.0/16 - A medium network with 65,536 addresses
The number after the slash (/) indicates how many bits are used for the network portion. The remaining bits are for host addresses.
Understanding the CIDR Suffix
IPv4 addresses are 32 bits long. The CIDR suffix tells you how many bits identify the network:
- /32 = 1 IP address (single host)
- /31 = 2 IP addresses
- /30 = 4 IP addresses
- /29 = 8 IP addresses
- /28 = 16 IP addresses
- /27 = 32 IP addresses
- /26 = 64 IP addresses
- /25 = 128 IP addresses
- /24 = 256 IP addresses (common for small networks)
- /16 = 65,536 IP addresses
- /8 = 16,777,216 IP addresses
Use our Subnet Calculator tool to instantly convert CIDR notation to IP ranges and calculate network details.
Why Was CIDR Created?
Before CIDR, IP addresses were divided into fixed "classes":
- Class A: /8 - Huge networks (16 million addresses)
- Class B: /16 - Large networks (65,536 addresses)
- Class C: /24 - Small networks (256 addresses)
This was wasteful. A company needing 1,000 addresses had to get a Class B (65,536 addresses), wasting over 64,000 addresses. CIDR allows networks of any size: /22 gives exactly 1,024 addresses.
Common CIDR Blocks
Private IP Ranges (RFC 1918)
- 10.0.0.0/8 - Class A private range
- 172.16.0.0/12 - Class B private range
- 192.168.0.0/16 - Class C private range
Special Use Addresses
- 127.0.0.0/8 - Loopback (localhost)
- 169.254.0.0/16 - Link-local (APIPA)
- 0.0.0.0/0 - Default route (all addresses)
How to Calculate CIDR Ranges
To calculate the number of addresses in a CIDR block, use this formula: 2^(32 - CIDR suffix)
- /24 = 2^(32-24) = 2^8 = 256 addresses
- /20 = 2^(32-20) = 2^12 = 4,096 addresses
- /16 = 2^(32-16) = 2^16 = 65,536 addresses
Remember: The first address is the network address and the last is the broadcast address. Usable host addresses are 2 fewer than the total.
CIDR in IPv6
IPv6 also uses CIDR notation, but with 128-bit addresses. A typical IPv6 allocation is /64, giving 18 quintillion addresses per network - enough for any foreseeable use.