The default subnet mask of an IP address 192.168.10.10 is ______.

A.
255.0.0.0

B.
255.255.0.0

C.
255.255.255.0

D.
255.255.255.255

Explanation:
192.168.10.10 is a Class C IP address. A Class C IP address uses the first 24 bits for network identification and the last 8 bits for host identification. The result is an IP address range from 192.0.0.1 through 223.225.225.225 and a default subnet mask of 255.255.255.0.
Incorrect Answers:
A: Class A IP addresses use the first 8 bits for network identification and the last 24 bits for host identification. The result is an IP address range from 0.0.0.0 through 127.255.255.255 and a default subnet mask of 255.0.0.0.
B: Class B IP addresses use the first 16 bits for network identification and the last 16 bits for host identification. The result is an IP address range from 128.0.0.0 through 191.255.255.255 and a default subnet mask of 255.255.0.0.
D: 255.255.255.255 is not a valid subnet mask as it uses all 32 bits for network identification, leaving zero bits for host addresses.
References:
David Groth and Toby Skandier, Network+ Study Guide (4th Edition), Sybex, Alameda CA, 2005, pp. 127-128.