CIDR notation and dotted decimal notation are two ways to represent subnet masks in networking, and each serves a unique purpose.

Dotted decimal notation expresses the subnet mask as four octets separated by dots, such as 255.255.255.0, where each octet represents 8 bits, making it easy to see each byte of the mask. This format was traditionally used in early networking and aligns well with IP addresses, but it can be less efficient when defining complex subnetting schemes.

In contrast, CIDR (Classless Inter-Domain Routing) notation represents the subnet mask with a slash followed by the number of bits in the network portion, such as /24 for a subnet mask of 255.255.255.0. CIDR notation is more flexible and efficient for modern networking, as it enables precise allocation of IP addresses by specifying any number of network bits, rather than adhering to fixed classes. This flexibility allows for variable-length subnet masking (VLSM), optimizing IP address distribution and supporting scalable network architectures, making CIDR the preferred format in modern IP addressing and subnetting practices.

This table demonstrates how the classful blocks map to CIDR ranges.

This table is a quick and easy reference for how each CIDR value maps directly to its counterpart subnet mask.