Hallasan is not one single hiking rule. The summit-access sections of Seongpanak and Gwaneumsa use Jeju's official Hallasan reservation system, while other major routes such as Eorimok, Yeongsil, Donnaeko, Seokguram and Eoseungsaengak are currently listed as usable without a summit reservation.
For a summit attempt, choose Seongpanak or Gwaneumsa first, then book the date through the official Hallasan system. The park's reservation notice says bookings for a month normally open at 09:00 on the first business day of the previous month. Availability can change as people reserve or cancel, so do not treat a screenshot as a guaranteed slot.
A reservation is only one part of the plan. Hallasan uses seasonal entry and turnaround controls so hikers can finish before dark, and weather can trigger partial or full closures. On the morning of the hike, check the official live trail-control page again even if your reservation is valid.
Do not build a tight flight or ferry connection after a summit day. Seongpanak and Gwaneumsa are long mountain routes, and the official trail pages estimate roughly a full-day round trip. Start with enough daylight and carry weather-appropriate layers, water and food.
If the summit route is closed or sold out, consider a non-summit Hallasan route rather than trying to enter a controlled section without authorization.