What Is Classless Inter-Domain Routing (CIDR) – A Complete Guide

In this lesson of Network Fundamentals Course, we will talk about Classless Inter-Domain Routing. You will get all the knowledge you need about it and it will help you to pass the course. When talking about CIDR, it is very important to understand what it actually is. CIDR is a method used with a purpose to allocate IP routing, and of course, IP addresses. CIDR was introduced for the first time in 1993 and its purpose was to replace the previous addressing architecture. That addressing architecture was used for classful networks of many designs on the internet.

What was the goal of creating the CIDR? It would definitely be slowing down the growth of all routable tables, and of course, the routers. It also helped to slow such a rapid exhaustion of IPv4 addresses.

We have already talked about IP addresses and how they have two groups of bits. The network’s prefix and the most significant bits are those two parts. Le’ts say that the classful network which was designed especially for IPv4 with a prefix of 8-bytes has the same result. It always results in the A, B, and C block (we also talked about those block in the previous lessons). What Classless Inter-Domain Routing does than is that it allocates the addresses space to all of the Internet providers. It does the same for end users or a bit boundary address.

CIDR has several concepts. First of all, it is always based on some variable-length of subnet masking, also known as VLSM. It is a technique and it allows such a specification to have the arbitrary-length prefixes. But, each day CIDR was becoming more and more advanced and it started a new method for IP addresses. It is known as a CIDR notation. Here, the routing prefix or even an address itself is written down in such a way where a suffix always indicates the right number of bits in the prefix. It may be, for example, 192.168.2.0/24 when it comes to IPv4 or 2001:db8::/32 when it comes to IPv6.

CIDR creates supernets by introducing such administrative process of the allocation of all address blocks which are meant for organizations that are always based on their own short-term projects. The supernets are advertised as aggregates and that is how they reduce the number of entries in the whole routing global table.

We will now talk about CIDR notation a bit. We have already mentioned the basics, but I would that you repeat more about the notation at the end of this lesson. So, notations are very important for you to understand them, and don’t worry, they aren’t complicated. The most important thing you need to know here is that they are a compact representation of some IP address. Also, you must know that it always contains some routing prefix. Also, there is always an IP address in CIDR notation, but also a slash character we have already shown in the example. Besides these two characteristics, CIDR notation has a decimal number too. That number is known as a traditional mask, and it always starts counting from 1 bit from the routing mask.

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