Recognizing the differences is pretty much just memorization. There is a system to the numbers but overall there is little rhyme or reason for the numbering, example the 2950 is a switch while the 2911 is a router. But as I said there is a system. Example there is the 2500 series followed buy the 2600, 2800, and finally the 2900. The numbers that follow the overall model usually indicates the slots in the router. Example of this is the 2911 has 1 NM slot and the 2921 has 2 NM slots, however the 2951 only has 2 NM slots again (It has a host of other features though). Another example of their system is the 6500 switch. If it is a 6506 it has 6 Slots, the 6909 has 9 slots, and you guessed it the 6913 has 13.
Simply put there are tricks to identifing the differences and the models, but mainly....its just memorization.
Hope this helps
