This tool checks if numbers in the range [2, 2599417] are prime numbers or not.
This is simply achieved by dividing the number to check by all relevant prime numbers.
If a number is not a prime number, it will display the smallest possible divisor. This does not mean that it cannot be divided by any other numbers.
This page uses a ~2 MB collection of prime numbers which can be downloaded in the prime numbers section of this website.