On Fri, 30 Jul 2004, John Keown wrote:
I am not sure what particular RFC defines the ip4 CIDR notation but I have
been assured by CommunigatePro developers that it does exist in the early
RFCs.
Read rfc 1518, 1519, and 1817.
These mostly describe how addresses be allocated than a particular
standard for requiring that CIDR formatted addressing always occur on
the network boundary.
It is stated that the correct format for any cidr notation is
xxx.yyy.zzz.kkk/mm where xxx.yyy.zzz.kkk starts on the appropiate boundry
for /mm. Although a masking on a non boundry ip address may give the correct
results one should do it.
CIDR notation is simple short hand for a decimal representation of a
binary netmask. Thus /24 is identical to 255.255.255.0.
It's not incorrect to specify that 192.168.1.4 has a netmask of
255.255.255.0, there for it's not incorrect to identify an IP address
within a network as 192.168.1.4/24.
If you're working in a network environment, setting up routing tables,
and such, then it is probably considered cleaner and easier to read
when networks are clearly defined by identifying the network boundary
and netmask, a la 192.168.1.0/24.
If you do the math, the network boundaries are implied by the netmask,
thus, you don't really need to specify them.
It is also what I have been taught. I also know that if you enter it into a
router as I did with an config import to a cisco border router it can play
havoc with routing tables and bgp. Our providers shut down our ds-3 as they
were causing routing problems throughout their system. That was the last
time I imported a router config file that was manually edited. Typo killed
us and play havoc with Level three.
SPF has nothing to do with routing tables. And most people don't use
CIDR to define their routing tables anyways, at least in a router
config. Cisco requires netmasks in some places and reverse netmasks in
others, a la "192.168.1.0 255.255.255.0" or "192.168.1.0 0.0.0.255", but
I've never seen anyone use CIDR notation in a cisco config. Of course,
I've never seen anyone specify anything other than a network boundary in
a routing table either. Which doesn't mean people haven't tried.
But then again, I've never used my border router to filter my mail, or
verify sender addresses either.
-Tim
--
There are 10 types of people on Earth. Those who understand binary, and those
who don't.