CIDR Notes

CIDR Notes

This is part of the Cisco Notes series on Mike’s World News.

This post refers to the Subnet Cheat Sheet published earlier today and is some notes about CIDR and subnetting.

CIDR stands for Classless Inter-Domain Routing and is what replaced the Old Class A, B, and C networks. It is an abbreviation of sorts for the subnet mask. Rather than writing something like:

IP Address of with a subnet mask of
you can write:

Each subnet has 2 special reserved addresses.  The first address is the subnet address and the last address is the broadcast address.  As you went through the Subnet Cheat Sheet, you may have noticed, for example, that /24 had 256 addresses, but only 254 addresses available.  For the address used in the last example, the subnet address would be and the broadcast address would be, giving the first usable address of and the last usable address of

The smallest network could have 2 devices.  However, because of the 2 special reserved addresses (broadcast and subnet address), the smallest subnet would need 4 addresses.  From the Subnet Cheat Sheet, you will notice that this is a /30 network.  A /30 Network gives you 4 IP addresses, with 2 usable.  The way I calculate the size of Subnet by CIDR is to start with a /30 with 4 addresses.  I take the number of addresses and I double it.  Each time I double it, the CIDR notation goes down by one.  A /29 gives you 8 addresses, /28 gives you 16 addresses, and so on.

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