While making an itinerary, many times, we do not find complete routes in domestic or international sectors. That does not mean that the travel agent should refuge his client that this trip is impossible. If there is no direct way, then there can be some connections through indirect routes, e.g., if a person wants to go from Port Blair (Andaman & Nicobar Islands) to Agatti (Lakshadweep), then there is no flight which serves these two places directly. In such a case, he should go from Port Blair to Chennai and from Chennai to Cochin and from Cochin to Agatti. Similarly, if a person wants to go from Bhuj (Gujarat) to Dibrugarh (Assam), then he will not find any airline which serves these two places. In such a case, he should go from Bhuj to Ahmedabad or Mumbai, then to Kolkata and from Kolkata to Guwahati and then to Dibrugarh.
It means if one itinerary includes some small places, then one will not find such a connection. In such a case, the itinerary consists of small to the large city airport, then a large airport to another large airport, then from large to a small airport. In it, the massive airport situated in liberal cities acts as a hub, and the small airport in small cities acts spoke of a wheel of the cycle.
This case is also right in the international itinerary, e.g., if a traveler wants to visit places of South America or Africa from Asia. Perhaps he will not get a direct connection. In such a case, he should find out links from some nodal hubs, e.g., London, Paris, or Rome.
Thus, whether it is the domestic sector or international sector, airlines follow the routes by ‘Hub & Spoke System’ whenever one makes air itinerary, one should remember this concept. Otherwise, the itinerary formulation will become very difficult or impossible.