From: Erick B. (erickbe@xxxxxxxxx)
Date: Sat Aug 10 2002 - 01:39:37 GMT-3
I think it is still valid to teach/mentor classful IP
routing, and forwarding techniques. I do network
support and do so for multi-vendor gear and shops.
Some vendors gear defaults to 'no ip classless' and
yes, it is an issue in many networks out there today.
I've solved network problems for a handful of
companies recently because they added some more
subnets, etc and then couldn't get places in their
network from some locations and this was the issue.
Simply turning on 'ip classless' or vendor X's word
for the same thing solved the issue. I have also seen
numerous cisco-shops where this has been a recent
issue.
Directed broadcast is also enabled on a number of
routers out there. There are numerous people running
11.x IOS still and using other vendors equipment with
older code versions that work just fine.
Depending on the vendor, upgrading to a current
version doesn't change these config options to todays
default values unless they default the config.
Erick B.
--- Wade Edwards <wade.edwards@powerupnetworks.com>
wrote:
> 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
>
> ========================================
> Dan Thorson - Seagate Technology, LLC
> desk +1 (952) 402-8293 fax +1 (952) 402-1007
> SeaTel 8-402-8293
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