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.).

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 all "1" and all "0" can't be used.

The 130.64.206.0 subnet has 255 potential hosts. Expressed in CIDR supernetting, it's /24 _(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? Take a look at the chart below. You'll notice that the CIDR number is the same as the number of 1's in the mask.