A ban is when a player has been banned or banished from a server by its admins. A banned player will no longer be able to return unless they either are unbanned by an admin or they access the server via a new IP.
In JKA, bans are handled by IP, this means that when a player is banned, their IP is added to a list of banned IPs, and anyone attempting to connect from that address will be unable to connect. For this reason, bans are not effective against players who have a Dynamic IP Address.
A range ban is a specific type of ban in JKA that removes an entire IP range (given to specific areas around the world) from accessing the server. It is achievable through the standard ban command
Range bans are used as a last resort to get rid of players that continuously break rules on servers and cannot be fully dealt with by a standard ban. A range ban works by blacklisting the range of the IP (example being 108, 99, etc). Any IP that has that specific range are then unable to access the server.
The obvious flaw to this method of banning is the fallout damage from executing this ban; Players whom have never broken any rules but are under the blacklisted range group are also banned from entering the server. The game engine does not allow for the removal of certain IP's under a range, so therefore the range has to be removed entirely from the banlist for that player to enter back into the server. This in turn also allows the offender to return to the server as well.
There is a possible chance now that the source codes have been released, that changes to the banning system can be made. Therefore, admins can potentially be able to use range bans without causing serious complication.