How is the subnet mask represented in CIDR notation?

The subnet mask in CIDR (Classless Inter-Domain Routing) notation is represented with a slash ("/") followed by the number of bits that are used for the network portion of the address. This method provides a concise way to denote the subnet mask by indicating how many bits in the 32-bit address are allocated for the network.

For example, a subnet mask of 255.255.255.0, which corresponds to eight bits for the subnet (or network), would be represented in CIDR notation as /24. This means that the first 24 bits of the address are designated for the network, and the remaining bits are available for host addresses. CIDR notation simplifies the representation and understanding of subnetting, making it easier for network professionals to quickly recognize the size of the subnet.

The other choices do not correctly represent the CIDR notation. A numerical value is too vague and does not specify the format directly. Using a dash is not standard in this context, and parentheses do not have any meaning in CIDR notation. Therefore, the structure established by the slash followed by the number of bits is the correct method of representation for subnet masks in CIDR notation.