Can you help me with Aircraft ID's?

Hi Everyone,

It’s my first post so the question may be a little niave but it’s something that I see regularly in the magazine and when I’m browsing pictures on the net.

If for instance I saw a photo of a Boeing 757-200 and it was listed as 757-223. Obviously the 757 is the type and the 2 is the 200 variant, but where does the 23 at the the end come from at the end. The registration was N177AN so it presumably has nothing to do with that, and this seems to happen on all types not just the 757, (I just used this example as it was handy)

Help in understanding this would be gratley appreciated!

Thanks

woodster757