The 24 bit address requirement goes back to the introduction of Mode S in 1987. Somewhere I have a Basic pgm that takes the N number and provides the 24 bit strapping, and vice versa. Other nations have their own numbering schemes, but the first digits represent the nation.
The aircraft i.d. in airplane types certified prior to about 1990 use 25 wires from each transponder to a shorting plug. Later planes have an aircraft i.d. module that transmits the i.d. to each transponder. Like ICAO says, they shall not be all ones or zeroes, and shall remain part of the aircraft registration.
TCAS will fault if the i.d. is all ones or zeroes. TCAS coordinates maneuvers with other TCAS equipped aircraft based on lowest number being in command.
GB