Final answer:
The network ID of an IP address is determined by the subnet mask, not by any fixed octet position. The subnet mask defines the division between the network and host portions of the address.
The correct answer is C.
Step-by-step explanation:
The question you're asking relates to the structure of an IP address and specifically what portion constitutes the network ID. An IP address is typically divided into two parts: the network part and the host part. The network ID can actually vary in length and is determined by the subnet mask, not strictly by any single octet.
However, in many cases, especially in classful networking, the network ID is referred to as the portion of the IP address before the subnet mask indicates the beginning of the host portion. For example, in a class C network where the subnet mask is 255.255.255.0, the first three octets constitute the network ID.
So, none of the options a), b), or d) universally identifies the network ID of an IP address. Therefore, the correct answer is c) The subnet mask determines the extent of the network ID within an IP address.