Besides from the fleet number on the nose mentioned above, on the very rear exterior of the aircraft is a alpha-numeric code starting with N. ex: N555US. This is the registration, and works just as well as the fleet no. Aircraft do have manufacturer's serial numbers (msn) and line numbers, which represent when this aircraft was produced. i.e. Line no. 1000 would represent roughly the 1000th airplane to be produced. Alternatively, above or near the passenger door on narrowbodies is the certificate of airworthiness or sanitary construction or something...This includes the registration number, along with some other info.