Buffer Overflow Intro. ©2002, Jedidiah R. Crandall, Susan L. Gerhart, Jan G. Hogle.  http://sfsecurity.pr.erau.edu
ANALOGY: Computer~~Mailroom
Imagine a mailroom with 256 mailboxes, numbered 0 through 255.
Each mailbox can contain only a single piece of paper with a number from 0 through 255 written on it.

Why 256?  Techie nomenclature:
One byte = 8 bits.  A bit can be either “0” or “1”. 
28 = 256 possible combinations of 8 bits. So every number can be represented.