CIDR Masks

CIDR (Classless Inter-Domain Routing)

Wondering what /20 means at the end of an IP address? The CIDR mask indicates the number of potential IP addresses or hosts in a subnet. It allows for a more flexible and efficient way to divide blocks of addresses than the old IP Class system (Class A, B, C, etc.). You aren't likely to ever interact with this DNS entry, but it's good to understand why clicking on 130.64.80.0/20 will take much lonnger to open than 130.64.215.0/28.

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Explanation

Let's use the most common example of a subnet mask, 255.255.255.0, as our starting point. Translated to it's binary equivalent, it's 11111111.11111111.11111111.00000000. When using this as a subnet mask, we're saying that every space with a "1" is the network mask (in other words, ignore these) and what's left of the IP address defines our hosts. That leaves us with a "potential" of 255 host IP addresses on this subnet.

• We say "potential, but in fact, there can only be 254 hosts in this net because 0 and 255 are not used in IP addresses.
• Subnet masks never have spaces. 11111111.11111111.11110111.00000000 (255.255.199.0) is not a valid mask.

The 130.64.206.0 subnet has 255 potential hosts. Expressed in CIDR supernetting, it's /24. Said another way, the first 24 bits of the address define the network portion of the address and what's left defines the host portion.

Still a little confused? Don't sweat the details. Just remember this: the CIDR number is the same as the number of 1's in the mask. Subtract the number of 1's from 32 and apply the number of 0's left to the the Binary Cheat Sheet below.

Binary Cheat Sheet

You can find an online subnet calculator at http://www.subnet-calculator.com/.