CIDR Block

A CIDR block (Classless Inter-Domain Routing block) is a range of IP addresses that is defined by a base IP address and a subnet mask (or prefix length). It is used for efficient IP address allocation and routing, replacing the older classful network design.

<IP address>/<prefix length>

For example, in the CIDR block 192.168.1.0/24:

• 192.168.1.0 is the base (starting) IP address.
• /24 is the prefix length, which indicates how many bits of the address represent the network part (in this case, 24 bits).

In this example:

• The subnet mask corresponding to /24 is 255.255.255.0.
• The range of IP addresses covered would be from 192.168.1.0 to 192.168.1.255 (256 addresses).

Point to remember:

• /24 means the first 24 bits are the network portion, and the last 8 bits are available for hosts, allowing up to 256 IP addresses in that range.
• The smaller the prefix length (e.g., /16), the larger the range of IP addresses.
• The larger the prefix length (e.g., /30), the smaller the range of IP addresses.