CIDR Set up

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Guest

Good Afternoon all,

I teach Active Directory at the local college. I was going over CIDR in
class using
192.68.24.24.33 using subnet mask of 255.255.255.224 I broke out the 30
possible hosts in the following ranges. 33-62, 65-94, 97-126, 129-158
161-190, and 193-222

A question was asked: why ranges 1-30 or range 225 - 254 would not work?
I was unable to explain that in an understandable way to the class. Any
help in explaining this would be helpful, and if they can be used I would
like to know that as well.
Thanks
 
Cochise said:
Good Afternoon all,

I teach Active Directory at the local college. I was going over CIDR in
class using
192.68.24.24.33 using subnet mask of 255.255.255.224
Click to expand...

Obviously you meant: 192.68.24.24.33
I broke out the 30
possible hosts in the following ranges. 33-62, 65-94, 97-126, 129-158
161-190, and 193-222

A question was asked: why ranges 1-30 or range 225 - 254 would not work?
Click to expand...

They would in almost all cases.
I was unable to explain that in an understandable way to the class. Any
help in explaining this would be helpful, and if they can be used I would
like to know that as well.
Click to expand...

In classical (e.g., non-CIDR) the low and high subnets were
disallowed. Some older routers and stations (or even older
routing protocols) that MAY still be in use do not support those
correctly.

Thus for conservative purposes those (low and high) subnets
should not be used UNLESS you absolutely need them OR
you can guarantee that all machines will function correctly on
the IP net while using them.

In general, if a router understands CIDR it probably does
support those low and high, but the mere fact of using the
notation does not guarantee this, nor does the fact that a
router supports the CIDR rules guarantee that all machines
do this as well.

Even Cisco declared this (formerly as least) on the entry
level CCNA (network associate) exam by saying these were
invalid for that exam. On the top-level CCIE exam these
nets are neither 'good' nor 'bad' automatically but the candidate
during the live exam portion may be asked to defend the use
or explain why these nets are invalid (or at least troublesome).

Example: RIPv1 cannot deal properly with these nets.
 
In
Cochise said:
Good Afternoon all,

I teach Active Directory at the local college. I was going over CIDR
in class using
192.68.24.24.33 using subnet mask of 255.255.255.224 I broke out the
30 possible hosts in the following ranges. 33-62, 65-94, 97-126,
129-158 161-190, and 193-222

A question was asked: why ranges 1-30 or range 225 - 254 would not
work? I was unable to explain that in an understandable way to the
class. Any help in explaining this would be helpful, and if they can
be used I would like to know that as well.
Thanks
Click to expand...

But they do work. Now they do, at least. I relay to my students a little
history on IP and the way they were inappropriately given out to companies
in the past, causing some of the current shortage issue, hence why
subnetting (an addendum RFC) came about in the late 80's. In the past (more
than 10 years ago), those ranges were not acceptable ranges. But due to
realization of IP shortages, the first and last ranges in each mask are now
accepted (another RFC). Routers and other equipment now accept it and have
been doing so for quite some time now. If you want the RFCs, I would have to
search and dig them up if I find some time.

It would be nice if the 127.0.0.0/8 range would be accepted since that
currently excludes 16,277,214 addresses (a whole /8 nework) and settle on
one specific IP as the Loopback. Matter of fact, if you look at some of the
older outdated exam preps (Nework +, for instance), say that range is not
acceptable, but the newer exam preps say they are acceptable. I've setup
many a network using the first range.

Hope that helps.

--
Regards,
Ace

Please direct all replies ONLY to the Microsoft public newsgroups
so all can benefit.

This posting is provided "AS-IS" with no warranties or guarantees
and confers no rights.

Ace Fekay, MCSE 2003 & 2000, MCSA 2003 & 2000, MCSE+I, MCT, MVP
Microsoft Windows MVP - Windows Server - Directory Services

Security Is Like An Onion, It Has Layers
HAM AND EGGS: A day's work for a chicken;
A lifetime commitment for a pig.
 
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