You are correct in the first assumption. This entry:

216.82.253.*~47795.0

will allow the entire Class C range to pass the greylist filter. While that list was not originally intended to be user-modifiable, it does support the use of wildcards. You can also use the "*" to wildcard larger ranges. For example:

216.82.2?3.*~47795.0

will allow you to exclude the range:

The decimal number after the "~" character indicates the number of days that have passed since 12/30/1899. The fractional part of the value is fraction of a 24 hour day that has elapsed.

So in your example, if you have an entry with:

  

   We have struggled to get the set of IPs that we need to include to the "GreyListAllowed", looking at the format it seems that if I type something like

216.82.253.*~47795.0

I'm letting the full Class "C" range to get included, and that happens to be what I need; so please tell me if I'm correct. 

Also, I want to know if I can type something like:

216.82.24*.*~47795.0

meaning that I'm allowing the following ranges:

216.82.240.*

While we do not know which provider does not honor the RFC that requires the specific server that made the initial connection to retry, I can provide you with a list of companies/providers that are associated with the IPs listed in the daily greylist file we update at http://www.logsat.com/SpamFilter/pub/GreyListAllowed.txt

The list is quite long, but if you'd like a one-time extract (the list is rather static as these IPs rarely change) please let us know and I can email it to you.

We have noticed that many servers are NOT honoring the mail standard for "retries" and when the SpamFilter rejects on first try, the sending server does not retry from the same IP, instead; relays the mail to another server; which in turns tries to connect and gets rejected by SpamFilter (cause is it's first time) and the story goes on and on until a server with an IP already in the "approved" GrayList takes the email and succesfully sends it.