What Is Classless Inter-Domain Routing (CIDR)? Meaning, Working, Advantages, and Disadvantages

Understanding Classless Inter-Domain Routing (CIDR)

CIDR or Classless Inter-Domain Routing is a specific method that combines several IP ranges in the network. This approach can also be called supernetting, and it is necessary for minimizing the size of the routing table and increasing the number of IPs within the enterprise networks.

A specific IP address is assigned to all the machines connected to the internet. With the help of this number, any endpoint can be tracked on the internet, and devices can communicate.

CIDR is needed for the efficient allocation of the IPs and creation of the certain flexibility in the networks. This method is used for the IPs’ allocation to guarantee data routing efficiency.   

How CIDR Functions

Prior to the discussions of all the little details about the CIDR, let’s start with the understanding of the IPs. They have 2 parts such as:

• Host address. This part consists of several numbers that are necessary for the identification of the device or host within the network.
• Network address. This number is necessary for network identification.

At the very beginning, the IPs were allocated only with the classful addressing system. The length of the address was rather specific, as well as a number of bits needed.  

Overview of Classful Addressing

Classful addressing, also known as IPv4, has 32 bits. Every number string is divided by the full stop and includes only 8 bits. With such a classful system, it was possible to choose between the following types of IPv4:

• Class A. This type is characterized by 8 network prefix bits. For instance, 87.2.0.2, where 87 is a network address and 2.0.2 a hosting address.
• Class B. Such type has 16 bits. For instance, 456.23.0.4 where 456.23 is a network address and 0.4 a host address.
• Class C. This category has 24 bits. For instance, in address 456.234.3.200, 456.234.3 is a network address and 200 a hosting address.

Introduction to Classless Addressing

The specifics of the classless addressing are in its usage of VLSM or variable length subnet masking. That means it is possible to change the ratio of bits in the host and network address. The subnet mask functions by dividing IPs into host bits and network bits.

With the help of VLSM sequence, it is possible to change IP space in the subnets of various sizes. Every subnet can have a different host count and IPs. Classless IP has a suffix value that specifies the number of bits to a usual IP. For example, 456.8.0.0/30 is an address; the first part (456.8.0) is a network address.  

Limitations of Classful Addressing

• Limited flexibility in IPs. This means that all the classful types of address could support only a certain number of devices. For instance, class B could support a little bit over 65,000 hosts; class A – over 16,000,000 hosts; and class C – 254 hosts.

Such a scenario became extremely inconvenient for lost of users. Just imagine a company with 300 devices that cannot use class C, so it uses class B and has lots of wasted IP space.

• Limitations in network design. In case there was a necessity to combine networks, such a possibility was unavailable for the users.  

How CIDR Works

CIDR functions by allowing network routers to direct packages of information to the needed device based on the subnet. The IPs aren’t categorized as in the classful approach, but host and network addresses are retrieved as specified in the CIDR suffix.

CIDR Block Allocation

The block is a range of IPs that are within one network prefix and have the same number of bits. So, a huge block has a small suffix and includes more IPs.

The process works the following way – IANA or Assigned Numbers Authority allocates huge blocks to RIR or regional registers. Then, regional registers allocate small blocks to LIR or local registers. The next phase is further allocation to the enterprise use. Individual users can apply for such blocks from the internet service provider.

Due to the CIDR, IPs are represented in binary. The blocks are extremely important because they enable address groups and use them as a single entity. This means that several IPs within the block have a sequence of bits.

When speaking about IPv4, in this case, CIDR blocks use syntax that is almost identical to IPv4. For instance, the IP could be 453.234.0.9. After the mentioned address, there should be a slash with the number that shows the shared bits.

CIDR blocks can also be used for IPv6; the syntax of such addresses is almost the same as in IPv4. However, the length of the prefix can be between 0 and 128. This number specifies the number of shared bits.

The blocks are extremely important for grouping and management of IPs. They also impact the simplification of the routing decisions.

As we have already mentioned, block assignment includes several steps, and a couple of organizations are involved in the process. So, let’s discuss these steps in detail.

Role of the Internet Assigned Numbers Authority (IANA)

Assigned numbers authority deals with allocating short/large prefix CIDR to the regional register. The largest possible block is /8, and it can include more than 16 million addresses.

Function of Regional Internet Registries (RIR)

After IANA, the next step is the regional register. Among the reginal registers, we can specify ARIN or RIPE NCC. They are responsible for the management of IPs depending on the specific geography. After receiving huge blocks from IANA, they assign small subnets at this stage.

Responsibilities of Local Internet Registries (LIR)

After regional registries, the following step is local registries that distribute IPs to the networks of users in the local areas. The size of the subnet is chosen based on the network needs.

ISPs and End-User Network Assignments

In case the network is served by one service, it obtains IP from the ISP. In the case scenarios when there are a couple of ISPs, then it is possible to receive provider-independent address space from the regional registers.

CIDR Blocks in IPv4

The subnets are used in CIDR for the representing of IP groupings. In the initial subnet address, all the bits are “zeros”. This first address functions as a network reservation. The last address includes “ones” for the bits. This creates a broadcast address for the network. This means individual users cannot access the first and last addresses in the network. All the other addresses are available for hosts.

For more efficiency in terms of space usage, smaller subnets are supported. The issue is that the subnets don’t give addresses because of broadcast addresses and reserved networks. The exception from this rule relates only to RFC 3021.

CIDR Blocks in IPv6

The major peculiarity of IPv6 compared with the previously discussed IPv4 is that it supports more IPs. IPv6 includes 8 hexadecimal value sets that are divided by colons.  CIRD blocks in IPv6 are needed for the address aggregation with various prefixes.

With such an approach, it is possible to have global route summarization and guarantee sufficient address pools. The standard subnet in IPv6 is a/64 block. This size is necessary for address autoconfiguration so that it is possible to configure addresses without the actual users’ involvement.

Advantages and Disadvantages of CIDR

Key Benefits of CIDR

• Minimized waste in terms of IP address space. With CIDR, it is possible to use a specific number of IPs and in such a way minimizing the wastage.
• Easy data transmission. The effective division of subnets, easy grouping, and creation are crucial for data transmission.
• The flexibility of supernet deployment that could not be achieved with standard masking architecture is a major plus.
• Creation of VPC. CIDR addresses are used as needed for the data transfer among the devices.  

Potential Drawbacks of CIDR

• Security. Without the tech skills, it might be a little bit more challenging to use some standard security measures.
• Complexity. Managing and implementation might not be the easiest process for the new admins.
• Compatibility. There are old network devices that might not be compatible with CIDR.

Summary and Final Thoughts

CIDR is an effective method of IP allocation that can greatly influence the minimization of space waste and improve efficiency. For more precise prefix specification, this approach supports VLSM. In addition, its users are getting great flexibility for the prefixes of any length. Sure, there are some disadvantages of this method as well, so it is better to consider both the pluses and minuses before the implementation.