From: Wade Edwards (wade.edwards@xxxxxxxxxxxxxxxxxxx)
Date: Fri Aug 09 2002 - 17:02:01 GMT-3
Unless this is a really really really old router then this is not
actually a bad practice anymore because it has to deal with Classful
routing and has no bearing upon modern, within the last decade,
networking.
I assume that since sending a packet to 10.255.255.255, on a modern
router, will only send a directed broadcast to the highest subnet on the
10-net, or be dropped as it should because of DDoS, then this is still
not a problem.
Does anyone know of any network equipment that still routes based on
classful networks, in production, with which this would be a problem?
The only reason why I bring this up is because some people that were
studying for their MCSE brought up to me that the top and bottom subnet,
all ones and all zeros, could not be used. I told them they were smoking
something because I use them in our production network. They showed me
in their study material that they state that they cannot be used. This
floored me that they should be preaching this non-sense to network
people. I never ran into a problem on our network and have never ran
into this problem so I just wanted to know if it was legacy crap that is
still being spread around or if there was an actual problem that could
arise from using this subnets.
IMHO this bad information should not be taught anymore. It's like saying
that programs, on an Intel platform, have to reside in 64K of memory.
This may have been true at one time but it is not the case anymore and
they have stopped saying this a while ago. So we should say, it used to
be considered bad practice but this is no longer the case.
L8r
-----Original Message-----
From: Dan.Thorson@seagate.com [mailto:Dan.Thorson@seagate.com]
Sent: Friday, August 09, 2002 1:41 PM
To: Wade Edwards
Cc: ccielab@groupstudy.com; Wright, Jeremy
Subject: RE: ip addr 10.0.0.254/24
Well, in your example of net-10 with a /24, the IP address
10.255.255.255
could be either of the following
1) the all-subnets-directed broadcast (all subnets in the classfull
network
10/8)
or
2) the subnet-directed broadcast for network 10.255.255.0/24
So the reason a subnet portion of all 1's is bad is pretty easy to see
(which is it, all subnets, or just one?).
On the other hand
10.0.0.0
in one case is the network number for the class-full network 10/8, but
is
also the network number for the 10.0.0/24 subnet. This could lead to
some
routing confusion, I suppose, if you don't keep track of subnet
masks....
... and as has been said previously, modern [Cisco] network equipment
can
keep track of the difference between these two routes (with ip
subnet-zero
enabled).
danT
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Dan Thorson - Seagate Technology, LLC
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SeaTel 8-402-8293
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