AboutThis 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 known prime numbers which are smaller than ½ of the number to check.
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, however!
For the technically-interested: This script uses a ~2 MB database that can be downloaded in the prime numbers section of this website.