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

[Dan Williams <dcbw redhat com>]

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

>      struct in_addr *prefix2netmask(int prefix) {
>          int mask = 0;
>          char *buf = NULL;
>          struct in_addr *ret;
> 
>          if ((buf = calloc(sizeof(char *), INET_ADDRSTRLEN + 1)) ==  
> NULL) {
>              return NULL;
>          }
> 
>          mask = htonl(~((1 << (32 - prefix)) - 1));
> 
>          if (inet_ntop(AF_INET, (struct in_addr *) &mask, buf,
>                        INET_ADDRSTRLEN) == NULL) {
>              return NULL;
>          }
> 
>          if ((ret = calloc(sizeof(struct in_addr), 1)) == NULL) {
>              return NULL;
>          }
> 
>          memcpy(ret, (struct in_addr *) &mask, sizeof(struct in_addr));
>          return ret;
>      }
> 
> And converting a netmask to a prefix in C (take a struct in_addr, give  
> back an int):
> 
>      int netmask2prefix(struct in_addr *netmask) {
>          int ret = -1;
>          struct in_addr mask;
> 
>          if (netmask == NULL) {
>              return -1;
>          }
> 
>          memcpy(&mask, netmask, sizeof(struct in_addr));
> 
>          while (mask.s_addr != 0) {
>              mask.s_addr = mask.s_addr >> 1;
>              ret++;
>          }
> 
>          return ret;
>      }
> 
> I pulled that code from a library a wrote...didn't just write it here,  
> so the API is based around what that library needs internally.  But  
> all of that is probably not useful to most people, so is there a  
> command that can convert between netmasks and prefixes?  Yes.  ipcalc:
> 
>      $ ipcalc -p 192.168.1.100 255.255.255.0
>      PREFIX=24
>      $ ipcalc -m 192.168.1.100/24
>      NETMASK=255.255.255.0
> 
> And it even gives it to you in shell env format so you can use it in  
> scripts.
>