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CIDR 101 FOR DUMMIES  
The Fundamentals An IPV4 address we are all familiar with looks like so XX.XX.XX.XX where XX is a number between 0 and 255. For example 127.0.0.0 or 10.0.0.1 Now, what if we wanted to express a range of IP addresses? How would we do that? We could do it like this: 10.0.0.010.0.0.255 which means all the IP numbers between 10.0.0.0 and 10.0.0.255 inclusive. But there is a shorter way to do it. Like this: 10.0.0.0/24 or even 10.0.0/24 OK, here's the arithmetic: an IP address is actually a binary number 32 bits long. To make it "more readable" we split the 32 bits into 4 groups of 8 bits like so Example #1: 10.0.0.1
Example #2: 192.168.0.15
Example #3: 255.255.255.255 (all 1's)
Do not read what follows if you don't get what was said above


What is a CIDR and what does it represent? In short, a CIDR is a shorthand method for representing a range of IP addresses. Here are some examples of CIDR Numbers:
How does 32/8 mean 32.0.0.0 to 32.255.255.255? We are agreed that an IP number is made up of 32 bits in binary format right? (See Examples #1, #2, #3 above). What 32/8 or 32.0.0.0/8 means (they are synonymous) is this:
Lets see what this looks like spelt out:


How does one calculate a CIDR? The answer depends on which side you work from  if you are trying to calculate an IP range from a CIDR or a CIDR from an IP range. Let's look at both:


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