Federal Airways
The federal airway system is the primary means of routing IFR traffic. Airways may be comprised of navaids, waypoints, fixes, and intersections. They are 8 NM wide, often stated as extending 4 miles to either side of the airway centerline. Airway courses are magnetic, and distances are nautical.
Victor airways extend from 1200 feet AGL, unless otherwise specified, up to but not including 18,000 feet MSL. They are designated by the letter V and a number, such as "V12". Jet airways, designated by a J and a number, extend from FL 180 up to FL 450. VORs are the principle navaids used to define airways.
A high altitude RNAV route is identified as a "Q" route, while a low altitude RNAV route is a "T" route. RNAV routes are depicted in blue.
Where prohibited airspace exists, the airway does not include the airspace of the prohibited area.
Just like highways, airways are typically given an odd number when they run north/south and an even number if they run east/west.
Random Routing
Pilots may also file random routing, instead of flying along federal airways. These pilot defined, or random, routes may be defined by points of latitude and longitude, radial and distance fixes from navaids, or offsets from an existing route. Random routes will only be approved by air traffic control when radar monitoring is available. Even with radar coverage, air traffic control may still need you on an airway or some other ATC preferred routing due to enroute traffic. In congested areas, it can quickly become very difficult for ATC to separate and organize traffic when everybody is on a random route.
If you file a random route, you should be prepared for ATC to place you on a published route, should traffic conditions change, equipment fail, or military airspace become active, for example. You could be cleared along an established route simply from the air traffic control perspective of keeping things locked down, in terms of procedures. In other words, they might just be trying to maintain pilot expectations of routing to avoid being inundated with fixing routes during busier times.
Even if given a longer than desirable route, pilots always have the option to request shortcuts enroute. Once you're in the air and in radar contact, controllers may have more ability to handle your more direct route.
Preferred IFR routing is listed in the A/FD. If you are staying in the low altitude structure and are not flying into or near major cities, filing the preferred IFR route is probably not necessary. Low altitude traffic is greatly reduced in recent years due to increased costs of flying. Whether the controller will assign you the preferred routing depends on the specifics of the facility and personalities of the personnel involved.
The North American Route Program
NRP routing is available to all traffic operating at or above FL 290. It is intended to allow operators to plan routes that are more efficient than other available routes. NRP aircraft are not subject to route restrictions, such as published preferred IFR routes beyond a 200 mile radius of their departure or destination airport.