[pycrypto] Distribution of prime numbers?

[Dwayne C. Litzenberger]

Can anyone point me to a resource that would help me answer the following 
question?

     Given a b-bit positive integer N, what is the probability that N is 
     prime?

     In other words: What is the a-priori probability of a number being 
     prime *before* we conduct any primality test?

-- 
Dwayne C. Litzenberger <dlitz at dlitz.net>
  Key-signing key   - 19E1 1FE8 B3CF F273 ED17  4A24 928C EC13 39C2 5CF7
  Annual key (2009) - C805 1746 397B 0202 2758  2821 58E0 894B 81D2 582E