Final answer:
The classful subnet mask for an IP address depends on the class (A, B, or C) of the address. Classes A, B, and C have default subnet masks of 255.0.0.0, 255.255.0.0, and 255.255.255.0, respectively. This mask helps in differentiating the network and host portions of the address.
Step-by-step explanation:
The subnet mask that would be used as the classful mask for an IP address depends on the class of the address. IP addresses are divided into five classes (A, B, C, D, and E). Class A addresses use a default subnet mask of 255.0.0.0, Class B addresses use a subnet mask of 255.255.0.0, and Class C addresses have a subnet mask of 255.255.255.0. Classes D and E are used for special purposes and do not have a classful subnet mask. To determine the class of an IP address, you need to look at the first octet (the first set of numbers) of the address.
If the first octet starts with a number between 1 and 126, it is a Class A address. If it begins with a number between 128 and 191, it is a Class B address. For numbers between 192 and 223, they are categorized as Class C addresses. Class D and Class E addresses are not normally used in standard networking environments. The subnet mask is crucial for identifying the network portion of an IP address from the host portion.