What attack cracks a password or encryption key by trying all possible valid combinations from a defined set of possibilities (a set of characters or hex values) ?

Brute Force attack.

Explanation:

A network is a platform where end user devices like computers are connected to communicate and share resources. There are public networks and private networks.

A public network has its end devices and servers configured with a public IP address, which is routable on the internet, while private networks uses private IP addresses which can be used on the internet.

Private networks can be made accessable to public users by configuring an authentication and authorization policy, which could be one or a multi factor authentication. These requires a password and other factors to access the services of a private network.

An attacker can easily access a one factor or a password accessible user account, if the password is weak by using the process called a brute Force attack.

The brute Force attack exploits the vulnerability of weak passwords by entering possible valid combination from a defined set of possibilities.