Login attacks

[Jonathan Berry]

On Tue, 7 Dec 2004 20:47:02 -0600, Mike Klinke <lsomike at futzin.com> wrote:
> On Tuesday 07 December 2004 20:04, Rick Stevens wrote:
> > > Another that bears blocking completely is 64.0.0.0/24 as its
> > > 100% spam of the non-edible variety.
> >
> > Whoa, buddy.  The entire 64.0.0.0/8 is NOT a spam source.  We
> > have a /19 in that space and we're not spammers.
>  
> Rick, note he said /24 (256 nodes) not /8 (16+ million nodes)
> 
> Regards, Mike Klinke

Hi,
I'm not familiar with this notation for IP ranges.  What exactly does
the "/24" or "/8" mean?  It's obviously some kind of mask.  How does
it correlate to a net mask?
Thanks,
Jonathan