What is CIDR and how to use CIDR notation in cloud networks

Just like area codes and phone numbers are regulated and assigned, IP addresses follow some rules to ensure they’re unique.

Up until 1993, the system used to allocate IP addresses was class-based. However, this system had limitations, and the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing, or CIDR, to solve that issue.

This was a game-changer as CIDR used a more flexible and variable way of allocating IP addresses, effectively tackling the scalability issues the older system couldn’t handle.

But what is CIDR? We’ll dive into the essentials of this classless method of IP address allocation and how to use CIDR notations in cloud networks.

It allows more flexibility in adding IP addresses and the number of IP addresses that can be assigned.

A CIDR address resembles a normal IP address but ends with a slash and a number. 

For example, an IPv4 CIDR address may look like this: 198.158.1.0/24, where 198.158.1.0 is the network prefix and /24 is the suffix indicating the number of bits of the prefix.

You can apply the same CIDR notation to IPv6 addresses. The only difference is IPv6 can have up to 128 bits, whereas IPv4 only has up to 32 bits.

The Internet Assigned Numbers Authority (IANA) allocates large blocks of IP addresses to Regional Internet Registries (RIRs) for large geographical areas like Europe and North America. 

RIRs then break down the CIDR blocks into smaller blocks, which they distribute to Local Internet Registries (LIRs). This subdivision continues until the IP addresses finally reach internet service providers (ISPs), who allocate them to end users. 

To better understand how CIDR works, let’s look at what IP addresses and subnet masks are.

What is an IP address and subnet mask?

An Internet Protocol (IP) address is a specific identifier given to a device on a network. With an IP address, devices can communicate with each other over the network.

In the context of IPv4, an IP address is divided into four octets. Each octet is separated by periods (e.g., 198.158.1.0) and ranges from 0 to 255.

The IP address consists of two parts. The first part is the network address, and the second is the host address. A subnet mask indicates how many bits in the IP address are assigned to the network.

For example:

IP Address: 198.158.1.0

Subnet mask: 255.255.255.0

To understand this concept better, we must break down the decimal numbers into binary format, as computers don’t read IP addresses in decimal format. They only understand numbers in binary bits.

Using the 8-bit octet chart below, we can find the binary numbers for the IP address and subnet mask.

Each bit in the octet can be either a 0 or a 1, and by manipulating the 1s and 0s in the octet, you can create a number range from 0 to 255.

The table below breaks down our IP address and subnet mask examples.

That results in the binary formats for each address.

The subnet mask’s first 24 bits have a value of one, which is equivalent to 255.255.255 in decimal form. This indicates that the first three octets of the IP address (198.158.1) represent the network address. The last octet (with all 0s in the subnet mask) represents the host addresses.

How did the classful IP addressing method work?

Using the classful method, IP addresses and subnet masks were initially divided into five classes (Class A, B, C, D, and E). However, two of these classes were for commercial use (D and E). 

That left three main IP address classes and default subnet masks.

Classful routing was limiting due to its rigid boundaries between classes, which often resulted in inefficient use of IP address space. 

For example, suppose a company has about 1,000 devices that need IP addresses. Under classful addressing, the organization can’t fit into a Class C network since it only supports up to 254 hosts. The next available class is Class B, which supports up to 65,534 hosts.

If the organization was allocated a Class C network, it would be limited to 254 usable IPs, which is insufficient for its needs.

On the other hand, if the company was allocated a Class B network, it would have far more IP addresses than it needed, leaving tens of thousands of IP addresses unused.

This lack of flexibility meant organizations often received more IP addresses than needed, leading to inefficient usage. The problem became worse as the internet grew and IPv4 addresses became scarce.

That overallocation led to the IETF’s introduction of CIDR IP addresses in 1993.

How CIDR solves the limitations of classful IP addressing

CIDR allows IP addresses to be allocated in blocks (CIDR blocks) of various sizes based on a company’s specific needs. The classless system uses VLSM to split an IP address into subnets of variable sizes, which is much more efficient.

In our example above, the company with 1,000 devices could be allocated a /22 (with a subnet mask of 255.255.252.0), providing approximately 1,022 usable IP addresses and eliminating the need to over-allocate in the old classful system.

Efficient use of IP addresses

IP addresses become more valuable as demand increases. CIDR helps alleviate scarcity by preventing over-allocation due to class limits. In particular, the jump between Class B and Class C (254 addresses to 65,534) was severe and led to drastic waste. 

However, with CIDR, you can precisely allocate IP addresses based on need rather than sticking to the fixed sizes the older class-based system dictated. For example, if an organization needs 500 IP addresses, CIDR can allocate a /23 network (which provides 510 usable addresses), ensuring minimal wastage.

CIDR flexibility helps you maximize IPv4 address space.

Improves routing efficiency

Speed is vital online, and CIDR helps speed up routing by letting you group multiple IP addresses into a single routing table entry, known as route aggregation or supernetting. 

For example, instead of having separate entries for 198.158.1.0/24, 198.158.2.0/24, 198.158.3.0/24, and 198.158.4.0/24, you can use CIDR to aggregate these into a single entry: 198.158.0.0/22. 

This dramatically reduces the size of routing tables and makes routers process data faster, contributing to overall improved network performance and speed.

Scalability

By eliminating the hard caps on IP addresses from the classful system, CIDR improves scalability by preventing the need for frequent restructuring. CIDR can handle many IP addresses and allocate them efficiently. 

For instance, as a company grows from needing 800 to 1,200 IP addresses, you can scale the network from a /22 CIDR block (up to 1,022 hosts) to a /21 CIDR block (up to 2,046 hosts) without needing a new IP address range.

In addition, you can scale your network incrementally with CIDR. Instead of making large jumps to the next class size, you can expand the network in smaller steps to preserve address space. For example, you can move from a /24 network to a /23 network, then to a /22, and so forth, as needed.

Simplifies network management

With CIDR, you can segment networks into smaller, more manageable blocks, which gives better control and oversight. For instance, separating a network into different CIDR blocks for various departments makes applying specific policies and restrictions easier.

The ability to arrange subnets in a hierarchical structure helps maintain the network structure effectively.

Challenges of CIDR implementation

While CIDR has many benefits, it also has its fair share of challenges when it comes to implementation, including compatibility. 

Training and skill requirements

Managing CIDR requires considerable networking knowledge, particularly in how IP addressing and subnetting work. 

For example, network professionals must know how to calculate and apply variable-length subnet masks, which differ significantly from the more straightforward, fixed masks used in classful addressing.

They also need to be proficient in binary math to calculate subnet sizes and boundaries. For instance, determining the appropriate subnet mask for a given number of hosts requires converting between binary and decimal formats as well as understanding how subnetting affects addressing.

Training or increasing IT staff size with already trained specialists can add to your costs.

Subnetting challenges

Subnetting (dividing a network into smaller parts) can be tricky and requires careful planning to ensure efficient use of IP addresses.

For instance, if you size a subnet improperly, it can lead to either a shortage or an excess of IP addresses. A common mistake is underestimating the future growth of a company, which leads to a situation where a subnet like /24 runs out of addresses and requires complex network restructuring.

In addition, diagnosing network issues becomes more challenging with multiple subnets. For example, a problem in one subnet, like a misconfigured /26 subnet, can have cascading effects on connectivity and performance. 

Therefore, network administrators must deeply understand how CIDR-based networks are structured to manage cloud networks effectively.

Network compatibility issues

Older network infrastructure and routing systems designed for classful IP addressing may not support CIDR adequately. For example, an older router might not understand CIDR notation like /25, leading to incorrect routing decisions or the inability to route traffic to those subnets effectively.

Similarly, you may experience routing challenges when interfacing with external networks, particularly those that still use classful addressing. Setting up efficient and accurate routing with external partners can be problematic if your network uses CIDR, but an external partner’s network doesn’t.

While necessary to overcome compatibility issues, upgrading network hardware and software can be resource-intensive, disruptive, and costly in the short run.

How to use CIDR notation in cloud networks

CIDR notation allows you to have flexible and efficient IP address allocations, which is vital for managing the complex and dynamic nature of cloud networks.

With that in mind, let’s look at how to use CIDR notation in cloud networks.

1. Understand your network requirements

The first step is knowing how many devices (like servers or computers) you plan to have in your network. This will determine the size of the CIDR blocks you need to ensure efficient network design.

For example, if you know your network will host 50 devices, using a CIDR notation like /26 (which supports up to 62 hosts) would be more efficient than the broader /24 (which supports 254 hosts).

2. Subnet your network

If your cloud network needs several sub-networks, you can divide your CIDR block into smaller blocks to reduce network congestion. 

When your network has all its devices in one large broadcast domain, you can experience a lot of broadcast traffic, slowing down the network. 

However, using CIDR to create subnets limits broadcast traffic to smaller domains, which helps maintain network performance. Think of it like organizing a city into districts to better manage traffic flow.

Subnets can also help organize the structure of your organization. For instance, departments like production and development can have subnets. The development team might use their subnet to test new ideas, while the production team uses their sub-network to handle the services you offer to customers.

3. Configure your network security

Configure security groups and access control lists using the CIDR blocks as you set up your cloud network. 

For instance, you can designate a /28 subnet that can host up to 14 devices and apply specific security policies to those devices.

Moreover, you can use CIDR to implement effective access control policies. For example, you might restrict access between the /24 subnet that your application servers use and the /26 subnet your administrative systems use. This way, only authorized traffic can flow between these critical parts of your network.

Similarly, configuring security groups and network Access Control Lists (ACLs) based on your CIDR blocks help filter unwanted traffic. For example, suppose you allocate a /24 subnet to a public-facing web application. In that case, you can set up ACLs to allow only HTTPS traffic to that subnet and block all other types of traffic, which can reduce the attack surface.

4. Monitor your network regularly

Regular network checks allow you to spot unusual patterns that might imply a security breach and proactively address the issues before they escalate. For example, if a /24 subnet allocated for web servers suddenly experiences a spike in traffic, it could indicate a DDoS attack or a need to scale your resources.

Likewise, if a particular subnet, say /26, designated for internal traffic, starts showing signs of external probing attempts, you’ll want to investigate immediately.

Keeping an eye on network traffic ensures that your network performs optimally, and you can identify underutilized or overutilized CIDR blocks. For instance, a subnet consistently at low capacity suggests that a smaller subnet could suffice, freeing up IP addresses for other uses.

Final thoughts: What is CIDR? Your guide to Classless Inter-Domain Routing

CIDR’s importance in modern networking cannot be overstated. It promotes efficient IP address usage, enhances routing processes, and offers unparalleled network flexibility.

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