CIDR Calculator Free Online Tool 2026

Last updated March 22, 2026

What Is CIDR Notation?

CIDR (Classless Inter-Domain Routing) notation is a compact way to describe an IP address range. It consists of an IP address followed by a slash and a prefix length — for example, 192.168.1.0/24. The prefix length (0–32 for IPv4) indicates how many bits of the address are the network portion; the remaining bits identify individual hosts within that network.

CIDR replaced the old class-based system (Class A/B/C) in 1993, allowing much more flexible allocation of IP address space. Today it's used everywhere: AWS VPC subnets, firewall rules, routing tables, and security group configurations all use CIDR notation.

How to Read a CIDR Block

For 192.168.1.0/24:

• The first 24 bits (192.168.1) are the network address — fixed for all hosts in this subnet.
• The remaining 8 bits are the host portion — giving 2⁸ = 256 total addresses.
• Subtract 2 (network address + broadcast) = 254 usable host addresses.
• Host range: 192.168.1.1 to 192.168.1.254.
• Broadcast: 192.168.1.255.

Common CIDR Blocks Reference

Private IP Ranges (RFC 1918)

Three address ranges are reserved for private networks and are never routed on the public internet:

• 10.0.0.0/8 — 16.7 million addresses, used by large enterprises and cloud VPCs.
• 172.16.0.0/12 — 1 million addresses (172.16.x.x through 172.31.x.x).
• 192.168.0.0/16 — 65,536 addresses, the most common home/office range.

Subnetting Example

Suppose you need to split 10.0.0.0/24 into 4 equal subnets. Borrowing 2 bits from the host portion gives you /26 subnets (64 addresses each, 62 usable):

10.0.0.0/26   → hosts 10.0.0.1   – 10.0.0.62   (broadcast .63)
10.0.0.64/26  → hosts 10.0.0.65  – 10.0.0.126  (broadcast .127)
10.0.0.128/26 → hosts 10.0.0.129 – 10.0.0.190  (broadcast .191)
10.0.0.192/26 → hosts 10.0.0.193 – 10.0.0.254  (broadcast .255)