.Airport i.d. Take your passport on all trips, even domestic flights and train journeys. The U.S. Federal Aviation Administration is now asking domestic passengers to carry some kind of government-issued photo identification, such as a driver's license. Some industry officials are pushing for rules that would require all passengers to carry their passports. "It has been taken for granted that because the United States is one country that it should be one travel free zone," wrote Robert Laney, president of 1stAir Inc., which books international business and leisure travel for clients worldwide. "However, if you compare it to the European Union, which also claims to be a free zone, it is not uncommon to have to show passports when traveling between countries. This should become a necessity when boarding any flight in the States." - Overcoming Airport Closures Although all U.S. airports except Reagan National outside Washington have reopened to commercial traffic since the attacks on Sept. 11, some international travelers expressed concern that there will be more flight disruptions and airport closures following the expected military retaliation by the United States. In such a case, industry sources offered some advice for anyone who must travel. If trying to reach the United States from Europe: Go through Canada. Or try to book a flight to Mexico City, with connecting flights to cities on the U.S.-Mexican border, such as as Tijuana or Juarez. Or book flights to the Caribbean on such carriers as British Airways or Virgin Atlantic, and take a charter or boat service to the U.S. mainland. If heading to the States from Asia: Since few direct flights to Canada from Asia are available, and none are offered to Mexico, travel from east to west. Reach a European hub and travel from there to Canada, Mexico or the Caribbean. For travelers trying to reach overseas destinations from the United States: Travel from Canada. Canadian hubs in Toronto, Montreal and Vancouver are less likely to experience the severe disruptions that will plague U.S. airports. Get current information on security and delays at specific airports by visiting each airport's Web site. Travel from the Caribbean: In the southeastern United States, book water transportation to the Bahamas, Antigua, etc. and take such carriers as British Airways or Virgin Atlantic. Use creative routing. Instead of trying to book a high-demand flight from Toronto to Paris, consider booking Toronto to Hong Kong to Paris. Don't be picky. ''Take whatever option is presented, and take it early,'' Laney said. - Packing Anthony Vandyck of Oxford in England sent the following tips: Follow the example of airline crews and take an overnight kit in your hand luggage with things for washing up, along with a change of clothes, even on short flights. You never know when you might be stranded. Take a copy of the Official Airlines Guide with you. The timetable can help you find an alternative flight if there are problems with yours. Take books and projects to keep you occupied in the case of delays or cancellations. Airlines allow Walkmans and other electronic devices to be used on aircraft, but be prepared for them to be examined by security. David Smith, who often travels from Europe to Africa and the States, suggested carrying enough travelers checks and cash, $200 to $1,000, so you can pay cash for a room in a city to which you might be diverted. Don't use luggage or tags that reveal your nationality. Be sure your mobile phone is fully charged and carry a spare battery. - E-tickets New restrictions on electronic tickets require passengers in the United States to show confirmation of their itinerary from the Web site or a boarding pass issued at the airport to get through security to boarding gates. Some aviation authorities and airlines in other parts of the world are following similar practices. ''We are sending mail out to all our customers with e-tickets to take additional documentation with them and proof of itinerary,'' wrote Simon Breakwell, managing director of Expedia Europe. ''These are the new regulations from Britain's Civil Aviation Authority. They seem to have followed the FAA guidance. For advice, call the CAA.'' Passengers are also advised to check with their airline before going to the airport. - Communication Set up an electronic mail account at www.hotmail.com, which is accessible worldwide from airports. The service is free and accounts are set up instantly. If phone lines are jammed you can use the Internet to tell your family and colleagues where you are and that you are safe. Contact business associates or friends and family at your destination to find out about the local conditions. Make sure they have the details of your itinerary. - Useful Web Sites The official site of the Federal Aviation Administration, www.faa.gov, publishes news, updates and answers to frequently asked questions. At www.quickaid.com one can find a directory of links to airport sites around the world. Links to hundreds of airline sites worldwide can be found at www.flyaow.com.