Which of the following characteristics defines a Class C IP address?

A Class C IP address is characterized by its subnet mask, which is typically 255.255.255.0. This subnet mask indicates that the first three octets of the IP address are used for network identification, while the last octet is used for host identification within that network. This structure allows for a significant number of networks (over 2 million) with each network supporting up to 254 hosts.

In the context of this question, the other options do not accurately represent the defining characteristics of a Class C address. A subnet mask of 255.255.0.0 is associated with Class B, which provides a larger network size and host capacity. The subnet mask of 255.0.0.0 falls under Class A, allowing for a single network with a vast number of hosts. Finally, a subnet mask of 255.255.255.255 is a special case used for broadcast addresses rather than defining a standard class of IP addressing.

Thus, the choice regarding a subnet mask of 255.255.255.0 correctly identifies the defining characteristic of a Class C IP address, illustrating its intended use for smaller, more numerous networks.