If you had space for only five digits on a piece of paper, the largest number you could write down would be 99,999. You’ve filled every spot with the largest digit available. What the Microsoft, air-traffic control, and Boeing systems all had in common is that they were 32-bit binary-number systems, which means the default is that the largest number they can write down is thirty-two 1s in binary, or 4,294,967,295 in base-10.
