Re: wha is ipv4 prefix?? (why not netmask)

[David Cantrell <dcantrell redhat com>]

On Aug 8, 2008, at 4:55 AM, Dan Williams wrote:

> On Thu, 2008-08-07 at 15:17 -1000, David Cantrell wrote:
> > On Aug 7, 2008, at 1:11 PM, Nathaniel McCallum wrote:
> >
> > > Derek Atkins wrote:
> > > > Miguel Angel Cañedo <mcanedo grupogcm com> writes:
> > > >
> > > >
> > > > > I was pulling my hair trying to set static ipv4 settings.
> > > > > Until I realized that NM 0.7 asks for PREFIX instead of  
> > > > > NETMASK....
> > > > >
> > > > > Now, my netmask should be 255.255.0.0
> > > > > How do I transalte that into the Prefix?
> > > >
> > > > /16 ?
> > > I also thought the wording was less-than-clear.
> >
> >
> > Prefix means CIDR prefix, which probably doesn't help clear it up
> > either.  People have been using classful IPv4 vocabulary for a long
> > time, but we don't really do that at a low level anymore.  Everything
> > is classless.  Being able to accept a subnet mask in dotted quad
> > notation is really a convenience thing for people still wanting to  
> > use
> > the pre-CIDR notation.
> >
> > This Wikipedia article does a reasonable job at explaining what  
> > CIDR is:
> >
> > 	http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing
> >
> > It also has a chart that lists all of the /XX values and what the
> > equivalent subnet mask is.  The /XX value refers to how many of the
> > leading bits of the 32-bit IPv4 address we want to be the same across
> > all of our hosts.  The most common would be /24, which is the old
> > Class C subnet with a mask of 255.255.255.0.  That means the first
> > three octets of the address will be the same for every host.  Only  
> > the
> > last octet is unique.  That gives you the possibility of having 256
> > unique hosts (in reality, it's 254 because .0 is the network address
> > and .255 is the broadcast address in IPv4-world).  See the table on
> > the wikipedia page for more details.
> >
> > Changing from prefix to netmask or netmask to prefix can be
> > challenging depending on where you need to do it.  Here's how to
> > convert a prefix to a netmask in C (take an int, give a struct  
> > in_addr):
>
> Can you look over the implementations of these functions in
> libnm-util/nm-utils.c and vet them for correctness?
>
> nm_utils_ip4_netmask_to_prefix
> nm_utils_ip4_prefix_to_netmask
>
> Thanks,
> Dan

Sure.

--
David Cantrell <dcantrell redhat com>
Red Hat / Honolulu, HI