What is "Hex"

[mypyrex]

Hello - just joined. Can somebody please tell me what is meant by Hex when the details of a flight come up? Thanks.

[Lasatalayas]

It's the Mode-S address of the aircraft in hexadecimal format.
This permits some 16,777,215 different idents.
A useful list can be found here http://www.mantma.co.uk/icao%20allocations.html