What is Proper CIDR Notation?
Quick Definition: CIDR (Classless Inter-Domain Routing) is a method used to efficiently allocate IP addresses on a network. CIDR allows network administrators to create subnets by allocating IP addresses based on size and need. This enhances the security, organization, and efficiency of the network.
CIDR is a critical concept needed for any network administrator or an aspiring one who intends to take the Network+ exam. CIDR also incorporates concepts such as IP addresses, subnetting, and route table management — all of which we will go over in detail. But first, let’s delve into CIDR notation and all its benefits.
What is CIDR Notation?
CIDR notation is a concise method to represent an IP address and its associated network prefix. The core part signifies the network prefix, while the CIDR suffix determines the variation or available addresses.
All IPv4 addresses are 32 bits, which can be broken up into 4 bytes, so the CIDR suffix informs the reader how many bits are already allocated. The full form of CIDR suffix will never be higher than 32 since that would be larger than the IP address itself.
What is CIDR Notation Syntax?
The concept of CIDR, network prefixes, and subnetting is fundamental knowledge required for exams like CCNA and Network+. Let’s break it down a little further with an illustration.
192.168.1.0/24
IP Address: Everything before the slash is the network IP address. All IP addresses will be a variation of the 192.168.1.X, where X is the host.
CIDR Notation: The slash and everything after it is the CIDR notation. The 24 represents the number of bits that are allocated to the IP addresses. Find the difference between 32 and 24 to determine how many bits are available for allocation.
Prefix: The network prefix is 192.168.0. These three octets are reserved and will remain unchanged throughout the IP allocation process.
Host: The host, part of the IP address ("0" in this example), is the section that identifies specific devices within the network. In this case, the last 8 bits (as determined by the "/24" CIDR notation) are available for different hosts on this network, allowing for 256 unique host addresses (2^8 = 256).
To bring the point home, the available IP addresses for this CIDR notation would be:
192.168.1.0
192.168.1.1
…
192.168.1.254
192.168.1.255
This is calculated by taking the CIDR suffix, subtracting it by 32, and then raising 2 to the power of that number.
Example: 32-24 = 8. 2^8 = 256. With a zero-based index, that is why it ends at 255. Therefore, there are 255 available IP addresses in this CIDR notation.
As a “fun” exercise, calculate the range of IP addresses for this one: 192.111.1.0/16. Remember, the smaller the number, the more IP addresses available.
What are the Benefits of CIDR Notation?
CIDR notation reduces the size of your routing table by grouping IP addresses by similar prefixes. CIDR’s convenient notation reduces the complexity of IP allocation and frees up network engineer’s time to work on other issues. It also facilitates Variable Length Subnet Masking (VLSM), which allows administrators to create subnets of varying sizes based on CIDR blocks. Ultimately, CIDR notations' core benefits rest on its ability to conveniently illustrate IP allocation.
Additionally, ISPs use CIDR notation for hierarchical addressing. They can easily and efficiently divide IP addresses in terms of global, regional, and local IP addresses by assigning CIDR notations with varying sizes. Global networks will have a high IP allocation, while local ones will have far less.
What are IP Addresses?
An IP address is a string of binary used to identify a host on a network. Each group of numbers is called an octet because it is composed of 8 bits. While IP addresses are binary numbers, it is often convenient and far more human-readable to write them in decimal notation.
For example, 192.168.1.1 is far easier to read than 11000000.10101000.00000001.00000001. With that said, let’s break down how to translate between the two.
While this is math-intensive, understanding the relationship between these two notations is vital. We’ll just look at the first octet, 11000000. Everything in computer science uses zero-based indexing, so first count out the total number of digits here. So, counting 0, it is seven. Start with the rightmost digit and count leftward. Then do the following method on each number, starting with the leftmost:
1 x 2^7 = 128
1 x 2^6 = 64
0 x 2^5 = 0
0 x 2^4 = 0
0 x 2^3 = 0
0 x 2^2 = 0
0 x 2 ^ 1 = 0
0 x 2^0 = 0
128 + 64 + 0 + 0 + 0 + 0 + 0 + 0 = 192. As you can see, that matches up perfectly with the leftmost octet. Let’s say you have the decimal representation and want to convert it to binary. Then dust off your math skills and apply the division-remainder method:
192÷2=96 with a remainder of 0
96÷2=48
96÷2=48 with a remainder of 0
48÷2=24
48÷2=24 with a remainder of 0
24÷2=12
24÷2=12 with a remainder of 0
12÷2=6
12÷2=6 with a remainder of 0
6÷2=3
6÷2=3 with a remainder of 0
3÷2=1
3÷2=1 with a remainder of 1
1÷2=0
1÷2=0 with a remainder of 1
This gives you 11000000.
IPv4 is still the most common IP format on the internet, but IPv6 will eventually supplant it. IPv4 is limited to around 4 billion IP addresses. While that may seem like a lot, it is actually limiting in the grand scheme of things. Think about everything with an IP address–especially with the advent of IoT. Even with subnet masking, this number is limited. IPv6, however, provides an astronomical amount of addresses, upwards of 340 undecillion. (That’s 340 followed by thirty-six zeros.) To illustrate, here is an example of an IPv6 address:
2001:0db8:85a3:0000:0000:8a2e:0370:7334
Notice how much bigger it is and how it is separated by colons instead of periods. IPv6 is bigger because it is 128 bits instead of 32 bits. That means it will change the calculation for CIDR notation. Instead of finding the difference between the 32 and the CIDR suffix, you will use 128.
What are the Basics of Subnetting?
Subnetting plays a critical role in segmenting a network. Subnetting is the process of dividing a larger network into smaller, more manageable components. Subnet masking is a critical component of subnetting, so let’s walk through that.
A subnet mask is a 32-bit number used in conjunction with an IP address that identifies the network and host portions of a network component. Let’s say you have an IP address of 192.68.75.0/24. The subnet mask would be 255.255.255.0, or in binary, 111111.111111.111111.000000. This notation provides an easy way for a network to determine which portions of an IP address can be allocated to a host.
Efficiently identifying subnet masks brings a host of benefits to the network. It can limit broadcast domains only to the IP addresses that fall in the subnet mask range. It also improves routing decisions by allowing routers to determine the most efficient way to route packets within a network. Instead of looking at an entire IP address, the router only needs to analyze the bits within the subnet mask.
Final Thoughts on CIDR Notations
CIDR notation is a powerful concept, appearing prominently on the CCNA and the Network+ exam. CIDR is a convenient notation that reduces the size of routing tables, increases network efficiency, and facilitates networking submasking. CIDR is directly related to IPv4 and IPv6 addresses. IPv4 is 32 bits, while IPv6 is 128.
I encourage you to log onto a router and review the router table. There will be plenty of examples of IP addresses using CIDR notation. Often, the best way to gain a conceptual understanding of a topic is to see it in action.
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