CIDR (Classes Inter-Domain Routing)

Last updated on Mar 15, 2023

CIDR (Classless Inter-Domain Routing) is an Internet addressing scheme that was developed to replace the classful addressing scheme, which had become inadequate due to the rapid growth of the Internet. CIDR is a hierarchical addressing scheme that allows for more efficient use of IP addresses, better routing, and easier administration.

In this article, we will explain CIDR in detail, including its history, features, benefits, and how it works.

History of CIDR

In the early days of the Internet, IP addresses were allocated using a classful addressing scheme, which divided the available IP address space into three classes: A, B, and C. Each class had a fixed number of network and host bits, which determined the maximum number of networks and hosts that could be created using that class of addresses.

The classful addressing scheme worked well in the early days of the Internet, when the number of connected devices was relatively small. However, as the Internet grew, it became clear that the classful addressing scheme was inadequate for several reasons:

It did not allow for efficient use of IP addresses: The fixed network and host bits allocated to each class meant that many addresses were wasted, as they could not be used by small networks or hosts that did not need all the bits allocated to them.
It did not allow for easy aggregation of routes: The fixed network and host bits allocated to each class meant that many small networks were allocated addresses from larger blocks, which made it difficult to aggregate routes.
It did not allow for easy allocation of addresses: The fixed network and host bits allocated to each class meant that many addresses were allocated to organizations that did not need them, while others ran out of addresses.

CIDR was developed to address these problems and to provide a more flexible and scalable addressing scheme for the Internet.

Features of CIDR

CIDR is a hierarchical addressing scheme that allows for more efficient use of IP addresses and better routing. Some of the key features of CIDR are:

Variable-length subnet masks: CIDR allows for the use of variable-length subnet masks, which means that networks can be allocated only the number of bits they need. This allows for more efficient use of IP addresses, as smaller networks can be allocated smaller blocks of addresses.
Aggregation of routes: CIDR allows for the aggregation of routes, which means that multiple smaller routes can be combined into a larger route. This reduces the number of routing table entries and makes routing more efficient.
Classless addressing: CIDR is a classless addressing scheme, which means that addresses are not allocated based on classes. Instead, networks can be allocated any number of bits they need, up to a maximum of 32 bits for IPv4 and 128 bits for IPv6.
Prefix lengths: CIDR uses prefix lengths to indicate the number of bits used for the network portion of the address. For example, a prefix length of /24 indicates that the first 24 bits of the address are used for the network portion, while the remaining 8 bits are used for the host portion.

Benefits of CIDR

CIDR provides several benefits over the classful addressing scheme, including:

More efficient use of IP addresses: CIDR allows for more efficient use of IP addresses, as networks can be allocated only the number of bits they need. This reduces the number of wasted addresses and allows for the creation of smaller networks.
Better routing: CIDR allows for the aggregation of routes, which reduces the number of routing table entries and makes routing more efficient. This reduces the load on routers and improves the performance of the Internet.
Easier administration: CIDR makes it easier to allocate and manage IP addresses, as networks can be allocated any number of bits they need. This reduces the amount of address space wasted on large networks that do not need all the addresses allocated to them, while making it easier to allocate addresses to smaller networks.

How CIDR Works

CIDR works by allowing networks to be allocated any number of bits they need, up to a maximum of 32 bits for IPv4 and 128 bits for IPv6. This allows for more efficient use of IP addresses, as networks can be allocated smaller blocks of addresses. CIDR uses prefix lengths to indicate the number of bits used for the network portion of the address. For example, a prefix length of /24 indicates that the first 24 bits of the address are used for the network portion, while the remaining 8 bits are used for the host portion.

CIDR also allows for the aggregation of routes, which reduces the number of routing table entries and makes routing more efficient. This is done by combining multiple smaller routes into a larger route, using a process known as route aggregation. For example, if two networks have adjacent address ranges, they can be combined into a larger route by specifying the common bits in the address range. This reduces the number of routing table entries and makes routing more efficient.

CIDR also allows for the use of variable-length subnet masks, which means that networks can be allocated only the number of bits they need. This allows for more efficient use of IP addresses, as smaller networks can be allocated smaller blocks of addresses. Variable-length subnet masks are specified using the prefix length notation, which indicates the number of bits used for the network portion of the address.

CIDR Examples

To understand how CIDR works, let's look at some examples.

Example 1: Allocating Addresses to Networks

Suppose we have a block of IP addresses, 192.168.0.0/16. This means that the first 16 bits of the address are used for the network portion, while the remaining 16 bits are used for the host portion.

We want to allocate addresses to two networks: Network A and Network B. Network A needs 1000 addresses, while Network B needs 2000 addresses.

To allocate addresses to these networks, we can use variable-length subnet masks. We can allocate a block of addresses to Network A using a prefix length of /22, which means that the first 22 bits of the address are used for the network portion, while the remaining 10 bits are used for the host portion. This allows for 1024 addresses, which is more than enough for Network A.

We can allocate a block of addresses to Network B using a prefix length of /21, which means that the first 21 bits of the address are used for the network portion, while the remaining 11 bits are used for the host portion. This allows for 2048 addresses, which is more than enough for Network B.

Example 2: Route Aggregation

Suppose we have three networks: Network A, Network B, and Network C. Network A has addresses in the range 10.0.0.0/24, Network B has addresses in the range 10.0.1.0/24, and Network C has addresses in the range 10.0.2.0/24.

To route traffic between these networks, we need to create routing table entries for each network. However, this can result in a large number of routing table entries, which can slow down routing and consume a large amount of memory.

To reduce the number of routing table entries, we can use route aggregation. We can combine the three networks into a single route by specifying the common bits in the address range. In this case, the common bits are 10.0.0.0/22, which means that the first 22 bits of the address are used for the network portion, while the remaining 10 bits are used for the host portion. This allows us to represent all three networks with a single route entry, which reduces the number of entries in the routing table.

Advantages of CIDR

CIDR offers several advantages over the previous class-based addressing scheme:

More efficient use of IP addresses: With CIDR, networks can be allocated any number of bits they need, up to a maximum of 32 bits for IPv4 and 128 bits for IPv6. This allows for more efficient use of IP addresses, as networks can be allocated smaller blocks of addresses.
More flexibility in network design: CIDR allows for the creation of networks of any size, which provides more flexibility in network design.
Reduced number of routing table entries: CIDR allows for the aggregation of routes, which reduces the number of routing table entries and makes routing more efficient.
Easier address allocation: CIDR makes it easier to allocate addresses to smaller networks, which reduces the need for large blocks of addresses.

Disadvantages of CIDR

CIDR also has some disadvantages:

Increased complexity: CIDR is more complex than the previous class-based addressing scheme, which can make it more difficult to understand and implement.
Incompatible with older routing protocols: Some older routing protocols are not compatible with CIDR, which can limit its usefulness in certain situations.
Increased risk of errors: CIDR requires more careful planning and management than the previous class-based addressing scheme, which can increase the risk of errors.

Conclusion

CIDR is a more flexible and efficient addressing scheme than the previous class-based addressing scheme. It allows for more efficient use of IP addresses, more flexibility in network design, and reduced routing table entries. However, it is also more complex than the previous scheme, and requires more careful planning and management. Despite its limitations, CIDR has become the standard for IP addressing and routing, and is an essential tool for managing modern computer networks.