CIDR (Classless Inter-Domain Routing) notation is a compact representation of an IP address and subnet mask. It is used to express the number of bits that make up the network portion of the IP address. CIDR notation is written using a forward slash (/) followed by the number of bits in the network portion of the address. For example, a CIDR notation of /24 means that the first 24 bits of the IP address are the network portion, and the remaining bits are the host portion.

There are several common CIDR subnet notations, each of which represents a different range of IP addresses:

/32: This notation represents a single IPv4 address. It is often used to specify a particular host on a network.

/24: This notation represents a Class C network with 256 addresses. It is often used in small-to-medium-sized networks.

/16: This notation represents a Class B network with 65,536 addresses. It is often used in larger networks.

/8: This notation represents a Class A network with 16,777,216 addresses. It is typically used for very large networks.

/0: This notation represents the entire IPv4 address space. It is rarely used in practice, as it allows any IP address to be used.

CIDR notation is also used for IPv6 addresses. The same principles apply, but the notation uses a colon instead of a period to separate the parts of the address. For example, a CIDR notation of /64 means that the first 64 bits of the IPv6 address are the network portion. IPv6 addresses typically use larger subnet sizes than IPv4 addresses, as the address space is much larger.