CIDR Notation Calculator – Subnet Planning Made Easy

CIDR Notation for Simple Internet Domain Routing, and yes, it sounds almost like “cider” when you say it out loud. In the 1980s Phil Karn created the Notation for compact display of an IP address together with its subnet mask and routing prefix. The idea itself is quite simple: you take a normal IP address, put a slash after it and add a number after it.

Do you know what that number really means? It shows how many bits of the address belong to the net itself.

CIDR Notation Made Simple

What does that number mean for you? It shows how many bits of the address relate to the network part. Take /24 as an example, that means that the first 24 bits belong to the net, while the remaining 8 bits stay for separate hosts.

Writing always 11111111.11111111.11111111.00000000 would be really mad, and so CIDR Notation helps as a short form. Instead of all those ones and zeros, you just write /24. The same mask equals 255.255.255.0, if you perfer decimal form.

CIDR Notation itself comes from counting the continuous chain of ones in the subnet mask, after you convert it to binary. Here the key point is, those bits go from the left to the right only. You can not simply mix ones and zeros freely.

For example, /25 becomes 11111111.11111111.11111111.10000000 in binary, which matches 255.255.255.128 in usual decimal Notation.

Looking at a practical case. For IP address 192.168.0.15 with mask 255.255.255.0, it becomes 192.168.0.15/24 in CIDR form. Here is another: 192.168.100.0/22 covers around 1024 IPv4 addresses, from 192.168.100.0 too 192.168.103.255.

But not every CIDR block that you find is correct. For example, 192.168.1.1/24 does not count, it should be 192.168.1.0/24 to be valid.

CIDR Notation shrinks the size of routing tables and makes nets much more efficient globally. It works for both IPv4 and IPv6 addresses. Programs like pfSense really prefer CIDR form instead of the old subnet mask.

Working with masks already became outdated in most current systems.

Say that you need around 600 IP addresses. One byte gives only 256. Doubling that, you get 512, and again to reach 1024.

That makes /22 your ideal choice, which in binary is 11111111.11111111.11111100.00000000. Learning binary math and converting between base 2 and base 10 really helps when you work with CIDR and subnets.

You commonly see 255.255.255.0 for /24 and 255.255.0.0 for /16. Old style addresses went away more than twenty years ago, fully replaced by CIDR as the current standard. CIDR blocks can describe ranges of IP addresses, and CIDR calculators help to convert between ranges and notations.

CIDR reallysaved the internet from IP address problems by means of clear mathematical logic.