The nine-digit ZIP code will increase the number of codes more than 500-fold, from 40,000 to 21 million for the nation's 80 million addresses.

The original, five-digit zip would have allowed 100,000 different numbers, but the Postal Service only allocated 40,000 at that time to allow room for expansion. The expansion has now taken the form of the nine-digit ZIP, in part to keep the first five digits the same.

The first three digits of both the old and new codes designate a regional center or main post office. The last two specify delivery stations and branches.

The final four digits, added after a hyphen, will break down the address even more exactly. The first two will define a sector, and the last two an even smaller unit called a segment.

Since the Office of ZIP Code Expansion was set up in 1978, its 10-member staff has worked with regional supervisors and local post offices to translate small geographical sections into a numbered system. The project, completed in 1980, cost $6 million.

The changes will make the current ZIP code directory swell to 40,000 pages. So the Postal Service plans to have a toll-free telephone number for ZIP code information, at a cost of $15 million for the first three years.