CIDR - What is it? How does it work?

     CIDR stands for Classless Inter-Domain Routing. The “Classless” in CIDR refers to removing the Class system of A, B, and C class IP addresses. CIDR’s purpose is to allow ISPs the ability to give users and companies an allotment of IP addresses. This keeps routing tables smaller while also increasing the amount of IP addresses available to a user or company. The addresses are allocated in block sizes and are noted in standard IP notation with a slash and number at the end. CIDR slash notation is also determined by the number of bits used in the IP address. In the bits, we are looking for and counting the number of 1s used in each octet. For example, the address, 255.255.0.0, has 16 bits that are 1s. That means its CIDR slash notation would be 255.255.0.0 /16. Let’s look at another example. 255.192.0.0 uses the first two octets, like our previous example, but 192 is smaller than 255, so we will have less 1s. 192 fills up the first octet with eight 1s and has two 1s in the second octet. This brings our 1s total to 10. So our CIDR slash notation would be 255.192.0.0 /10. Although CIDR removes the class system from most of the process, there are still certain CIDR values that only particular class address can use. The /8 through /15 can only be used with Class A network addresses. /16 through /23 can be used by Class A and B network addresses. /24 through /30 can be used by Class A, B, and C network addresses. This is a big reason why most companies use Class A network addresses. Since they can use all subnet masks, they get the maximum flexibility in network design (Lammle, 2020).  It should also be mentioned that the /30 is the largest CIDR notation that can be used as there has to be two IP addresses left free, the subnet and broadcast addresses. 

Here is a table that lists IP addresses and their corresponding CIDR values: 

Sources:

IP Addressing Guide. (n.d.). CIDR to IPv4 address range utility tool | ipaddressguide. https://www.ipaddressguide.com/cidr

Lammle, T. (2020). CCNA Certification Study Guide. volume 2, exam 200-301. Sybex.