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What Are CIDR and Subnet Masks?

CIDR uses notation such as `192.168.1.0/24` to represent an IP range. The number after the slash is the network prefix length.

One-line Explanation

CIDR is a compact way to describe an IP address range, where `/24` or `/16` controls how many addresses are included.

When You See It

When configuring cloud security groups, firewall allowlists, trusted proxies, or database access.
When planning private networks, VPNs, container networks, or office subnets.
When checking whether an IP address belongs to a certain network range.

How It Works

An IPv4 address has 32 bits. The CIDR number tells how many leading bits are the network part.

`/24` means the first 24 bits are fixed and the last 8 can vary, usually covering 256 addresses.

A subnet mask is another representation. `/24` corresponds to `255.255.255.0`.

Examples

Common private subnet

This usually covers 192.168.1.0 through 192.168.1.255.

192.168.1.0/24

Common Misunderstandings

Not every address in a range is always usable by hosts; network and broadcast addresses can have special roles.
`/32` means a single IPv4 address, not a large subnet.
Do not use overly broad CIDR ranges in public allowlists unless you really intend to expose that range.

FAQ