steve@dmnhack.uucp (Steve Pozgaj) (06/09/89)
The reason for NNX vs. XXX or NNN is that the N's designate one of a set of specific digits, while the X's designate *any* digit. Thus, a phone number is stated as NPA-NNX, with no care to the last 4 digits, since they can be anything. The N's just mean "more important to watch how these are selected", while the X's mean "pick whatever you want". The NPA (Number Plan Area code) is *real* special, so has its own name. Of course, N1X and N0X do convey the spirit of "pick the first one carefully; the middle one must be 0 or 1, and the last one's no big deal. (At least that's what I was told in a telephony course when I worked at Northern Telecom some 12 years ago.)
ron@ron.rutgers.edu (Ron Natalie) (06/13/89)
The numbering plan as found in Bell's Engineering and Operation
in the Bell System book explicitly lists N as { 2,3,4,5,6,7,8,9 }
rather than "pick this number carefully." The description of the
plan shows the transition from the N{0,1}X-NNX numbering plan to
the (then) future generic XXX-XXX numbering.
-Ron