How would I go about assigning unique ID's to each player?

I want to assign unique ID's for each player to be stored in a scoreboard. I'm using the ID's for reference later. I can't seem to figure out a way to go through each player, one by one, to assign a number in ascending order. For example: player_1 would be assigned to ID 1, player_2 would be assigned to ID 2, ... (Stored in ID scoreboard). How would I go about doing this?