Possible bug: pg_hba.conf file

Hey guys,

I'm a fairly new user to the postgresql world... so *perhaps* this is not a bug. But I cannot get
the following entry to work in the pg_hba.conf file:

host    all   all     0.0.0.0/0     md5   <-- doesn't work

host    all   all     4.0.0.0/8     md5   <-- matches anything 4.x.x.x (works correctly)

My intent is to recognize any external IP, and if they can supply a correct user/password then they
can get access.  So I'm guessing there is something wrong with the way 0.0.0.0/0 is being
recognized?

Thanks for you help!
Tyson Thomson