Question 1

What is the difference between CIDR and subnet mask notation?

Accepted Answer

They express the same thing in different formats. /24 and 255.255.255.0 are identical -- both mean the first 24 bits identify the network. CIDR notation is shorter and more common in modern configurations, while subnet mask notation is still used in older router interfaces and Windows network settings.

Question 2

How do I figure out which CIDR prefix I need?

Accepted Answer

Count how many hosts you need, then find the smallest prefix that fits. For 50 hosts, you need at least 52 addresses (50 hosts + network + broadcast), so a /26 (64 addresses, 62 usable) works. For 200 hosts, you need a /24 (256 addresses, 254 usable).

Question 3

What is a wildcard mask and when do I use it?

Accepted Answer

A wildcard mask is the inverse of the subnet mask. Where the subnet mask has 1s, the wildcard has 0s and vice versa. Cisco routers use wildcard masks in access control lists (ACLs) and OSPF area definitions. For a /24, the wildcard mask is 0.0.0.255.

Question 4

Can I use this for IPv6?

Accepted Answer

This calculator handles IPv4 addresses only. IPv6 uses a 128-bit address space with a different notation system. The CIDR concept still applies to IPv6, but the prefix lengths range from /0 to /128 instead of /0 to /32.

CIDR Calculator

Frequently Asked Questions