On Sun, 23 Mar 2003 08:13:38 -0800 "Vern W Heesch" <[EMAIL PROTECTED]> wrote:
> Ugg, forget that. It was too early to think. What I gave was for
> x.x.x.x/16
Yep. And I was just about to answer that.
For those of you who have been around networking for less than about 10
years, we didn't always use VLSM (variable length subnet masking), which
as I said is a subset of CIDR (classless inter-domain routing). We used
to use classful routing which is still built into ifconfig:
class A: first octet 0-127 (netmask 255.0.0.0)
class B: first octet 128-191 (netmask 255.255.0.0)
class C: first octet 192-223 (netmask 255.255.255.0)
class D: first octet 224-254 (experimental block, 224.x.x.x now used for
multicast for those paying close attention)
while the above is obsolete, if you configure an address like
64.116.181.3/29 (netmask 255.255.255.248), you must specify the
broadcast address or ifconfig will assume a broadcast address of
64.255.255.255. Obviously incorrect, since our broadcast address is
64.116.181.7.
Now VLSM is nice because it allows us to split things up neatly between
network and host addresses, the first non-255 octet being one of 0, 128,
192, 224, 240, 248, or 252 (or 254 if the second or third, but not last,
octet). With VLSM we can use the /# notation. (Yes, I'm ignoring the
special case /32).
With non-VLSM CIDR, we can't use /#. We will also get very large
headaches trying to calculate which IPs are found on a network with
absurd netmasks like 255.255.255.123. If you don't think this is valid,
you can try it on your network and see that it works just fine with the
following values:
network: 192.168.0.1
netmask: 255.255.255.123
broadcast: 192.168.0.133
hosts: 192.168.0.5, 192.168.0.129
yes, for this particular netmask, there are only 2 hosts, other non-VLSM
netmasks give varying numbers of hosts in different patterns scattered
about between the network and broadcast numbers.
Note: since I'm not a masochist and don't like headaches (or anding
binary netmasks w/ addresses), I use a program to calculate the above
just as the kernel does.
Ciao,
David A. Bandel
--
Focus on the dream, not the competition.
Nemesis Racing Team motto
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