In the military system, all odd-numbered airplanes are those whose primary mission is interception of enemy a/c (i.e. MiG-25, Su-27), even-numbered a/c are those which primarily serve for bombing and ground attack purposes (i.e. Su-20, Su-22, Su-24). There are exceptions, such as the Su-25 and the Su-30. This obviously does not apply to transports, although they all seem to have even numbers, except the An-225.