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CIDR notation

When dealing with networking, you often need to reason about ranges of IPs, such as "all IP addresses between 172.31.0.0 and 172.31.255.255" (there are 65,536 IP addresses in this range). The de facto standard for representing IP address ranges is called Classless Inter-Domain Routing (CIDR) notation. For example, the same 65,536 IP addresses can be represented in CIDR notation as 172.31.0.0/16. This notation includes the IP address (172.31.0.0) and the number of bits in the mask (/16). To understand what the notation means, you:

  1. Convert the IP address to binary: e.g., 172.31.0.0 in binary is 10101100.00011111.00000000.00000000.

  2. The mask tells you how many bits of the binary IP address identify the network (and stay constant for everything in that network) and how many bits identify unique hosts (and therefore, can vary). For a /16 mask, the left-most 16 bits stay constant, while the right-most 16 bits are allowed to vary.

  3. Putting that together, 172.31.0.0/16 represents all IP addresses from 10101100.00011111.00000000.00000000 (172.31.0.0) to 10101100.00011111.11111111.11111111 (172.31.255.255).

A few handy notes:

CIDR calculators​

You can use online CIDR calculators to quickly do the math for you.

CIDR blocks​

IP addresses expressed in CIDR notation are often called CIDR Blocks.

All IPs​

The CIDR Block 0.0.0.0/0 corresponds to all IP address.

Single IPs​

To specify a single IP address (e.g., the IP of a specific server), use the /32 mask: e.g., 4.4.4.4/32 is the CIDR notation for just one IP, 4.4.4.4.