What two principal network access methods are commonly used?

The principal network access methods that are commonly used in networking are based on how data packets are transmitted over a network medium. Contention and token passing represent two fundamental approaches for managing access to a shared communication medium.

Contention refers to a method where devices compete for access to the network; for example, in Carrier Sense Multiple Access with Collision Detection (CSMA/CD), devices listen to the network before transmitting. If two devices transmit simultaneously, a collision occurs, and both devices must wait a random amount of time before attempting to transmit again. This method is commonly used in Ethernet networks.

Token passing, on the other hand, uses a special control packet called a "token." Only the device in possession of the token is allowed to transmit data on the network. This method helps to eliminate collisions and manage network access more orderly, making it suitable for certain types of networks, such as Token Ring networks.

These two methods are critical in determining how various devices on a network can communicate efficiently, manage collisions, and ensure orderly access to the shared medium. The use of contention and token passing illustrates the fundamental ways nodes control access to a network, making this the correct answer.