Do you need a ready-made sequence or an algorithm for generating a number sequence (7 characters), with the ability to check for belonging to it?

Question

Algorithms

I am transmitting some device code using an unreliable protocol (IR). I can send a number from 0 to 4000000. I can't insert a checksum. I decided to use sequences in this range, where you can easily check whether a number belongs to this sequence and in this way check the validity of the received code. You don't need a lot of numbers, 256 is enough, but more is better.