What is CIDR?

Before we can discuss CIDR (Classless Inter-Domain Routing), we need to understand what IP address is, how it is divided (subnetting), and what versions of IP addresses are available. Currently, there are IPv4 (version 4) and IPv6 (version 6) addresses. IPv4 was introduced in 1980, and it is a 32-bit number having a total of about 4.3 million addresses. With the prosperity of the Internet and smart devices joining the network, 4.3 million IP addresses weren't enough to connect all devices available worldwide, and hence 128-bit IPv6 address was invented in 1998. For each computer, smartphone, tablet, and IoT devices joining the Internet, the number of IP addresses available within IPv4 wasn't enough to connect all devices.

The traditional IPv4 address was classified as class A, class B and class C; and entire block of class A, class B or class C IP addresses were assigned to ISPs and corporations. The classful routing system was inefficient in allocating IP addresses amongst organizations as class A, class B, and class C blocks are a predefined set of IP addresses that cannot further break down into smaller units. The classful address blocks are defined as follows:

• Class A: 8-bit network address and 24-bit host address having 16,777,216 IP addresses.
• Class B: 16-bit network address and 16-bit host address having 65,534 IP addresses.
• Class C: 24-bit network address and 8-bit host address having 256 IP addresses.

As you can see from the above, the entire 16,777,216 IP addresses are assigned to a single organization registering a class A network. This is a very inefficient allocation of IP addresses, especially when more and more devices are joining the Internet. To efficiently allocate the IP addresses into smaller blocks, CIDR (Classless Inter-Domain Routing) was invented in 1993. This is 5 years before IPv6 was introduced.

What is CIDR?

The introduction of CIDR extended the lifespan of IPv4 as well as the growth of routing tables. CIDR is only applicable to IPv4 addresses, and significantly improves the allocation of IP blocks in smaller chunks. For example, if a small business requires only 6 IP addresses, the smallest allocation in the classful system would be a class C, assigning 255 IP addresses to this organization instead of 6. With the CIDR allocation system, a block of 8 IP addresses would be assigned to this small business, improving IP allocation.

How does CIDR work?

When setting up a network, a subnet mask is used to break the IP address into a network address and a host address. The 32-bit IP address is divided into 2 parts: the first part is the network address, and the tail end is the host address. The subnet mask is used to mask the host address, thereby revealing the network address. CIDR uses variable-length subnet masking (VLSM), allowing it to mask any number of bits rather than 8-bit chunks in a classful system. CIDR notation is used in the following manner: Network Address / # of bits used in the network. For example, 192.168.1.0/24 is identical to a class A network beginning at 192.168.1.0. The /24 is class C, /25 would be half of that, which is 128 hosts, /26 would be 64 hosts in a subnetwork.

The table below is a reproduction of the CIDR network table provided in the various articles within our website.

Note: (1) 127 Network Address reserved for loopback test. (2) Class D (224-247, Multicast) and Class E (248-255, Experimental) are not intended to be used in public operation.