 We have a set of n values or n items and we randomly select one of those from that set. Then the probability of selecting a specific value is 1 divided by n. In this case, random, we mean that it follows a uniform distribution in that equal probability is selecting any of the possible values. As a first example, let's say we have a box with five color balls, red, green, blue, yellow and black. So we have a set of five items and we select a ball from the box and we do so randomly. So we put our hand in the box and grab a ball. Then the probability of selecting a ball which is yellow is 1 divided by 5. Similar, the probability of selecting a red ball is also 1 divided by 5 or 20%. As a second example, consider IEEE 802.11 Wi-Fi. The medium access control protocol includes a feature called the distributed coordination function or DCF. In that, our works is that each computer must as part of the algorithm select a random time to wait called a random back-off. What they do is they select a random number between 0 and 15 inclusive and that indicates how long that they need to wait. For example, if they select 3, then they have to wait 3 time slots before they get to transmit or attempt to transmit. So in this case, what's the probability that a particular back-off value is chosen? So the probability, for example, that the back-off chosen by a particular station or computer is say 3. Then we're selecting from a set of 16 values, 0 to 15 inclusive. So the probability of selecting a back-off of 3 is 1 divided by 16. Similar, the probability of selecting a back-off of any particular value is 1 divided by 16.