From: Durkin, Michael (MED US) (michael.durkin@siemens.com)
Date: Tue Jan 02 2007 - 14:59:08 ART
Hello (Eva I presume),
The way I think of it, and this is certainly subject to my own
intellectual limitations, CIDR is sort of the opposite of Subnetting.
With Subnetting, you are attempting to break an address space into more
subnets by sliding the bit mask to the right. With CIDR, you are doing
the exact opposite action, creating larger subnets by sliding the mask
to the left. This is useful for claiming unused addresses, and for route
summarization. The big payoff is smaller routing tables. CIDR was
primarily adopted to solve internet routing problems, but can also be
used on an internal network.
Consider the following example:
You have the following subnets in your organization: 192.168.1.0/24
through 192.168.127.0/24 for a total of 127 classful subnets. Your
company buys another company and wants to integrate your networks. Lucky
for you, they don't have any overlapping address spaces used internally,
so you can share routing information with little complexity.
Without CIDR, you would advertise all 127 networks into their routing
infrastructure. Yucky, what a mess. Why should you do that when they
really only need to know one summarized, or should I say aggregated
route since all your networks can be rolled up into one CIDR network.
In the example above, a CIDR capable routing protocol could advertise
your networks with 192.168.0.0 /17 which would include all your subnets,
as one CIDR address, but nothing outside your subnets would be included.
The resulting CIDR network would be 192.168.0.0/17.
For the first 127 subnets, they all have the same network address from
an outside network. Below I have copied in the first two subnets
(192.168.1.0 and 192.168.2.0) and the last two (192.168.126.0 and
192.168.127.0) from a subnet calculator for comparison. You will notice
that when using CIDR, they will all be advertised as the same network as
indicated by the subnet and subnet address. I also pasted in the next
CIDR block of address using the same prefix and 17 bit mask beginning
with the 192.168.128.0 subnet.
The first CIDR block therefore contains hosts (which would include all
routers and switches that fall in this range) within 192.168.0.0/17
through 192.168.127.254/17 range. The second CIDR block contains the
hosts within the 192.168.128.0/17 through 192.168.255.254/17 range. In
the example I gave above, you would only care about the first range, as
the second range falls outside your networks.
Subnet Subnet Mask Host range of CIDR Subnet
Broadcast address
192.168.0.0 255.255.128.0 192.168.0.1 to 192.168.127.254
192.168.127.255
192.168.128.0 255.255.128.0 192.168.128.1 to 192.168.255.254
192.168.255.255
-------------------------------------------------------
192.168.1.0/24 Network with /17 CIDR
IP Address : 192.168.1.0
Address Class : Classless /17
Network Address : 192.168.0.0
Subnet Address : 192.168.0.0
Subnet Mask : 255.255.128.0
-------------------------------------------------
192.168.2.0/24 Network with /17 CIDR
IP Address : 192.168.2.0
Address Class : Classless /17
Network Address : 192.168.0.0
Subnet Address : 192.168.0.0
Subnet Mask : 255.255.128.0
-------------------------------------------------
192.168.126.0/24 Network with /17 CIDR
IP Address : 192.168.126.0
Address Class : Classless /17
Network Address : 192.168.0.0
Subnet Address : 192.168.0.0
Subnet Mask : 255.255.128.0
-------------------------------------------------
192.168.127.0/24 Network with /17 CIDR
IP Address : 192.168.127.0
Address Class : Classless /17
Network Address : 192.168.0.0
Subnet Address : 192.168.0.0
Subnet Mask : 255.255.128.0
-------------------------------------------------
192.168.128.0/24 Network with /17 CIDR
Notice this has now moved into the 192.168.128.0 subnet, which is the
next CIDR block in this example.
IP Address : 192.168.128.0
Address Class : Classless /17
Network Address : 192.168.128.0
Subnet Address : 192.168.128.0
Subnet Mask : 255.255.128.0
I hope this helps. If not, you may find this website helpful, though it
really isn't a CCIE level explanation.
http://compnetworking.about.com/od/workingwithipaddresses/l/aa021003a.ht
m
Mike
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Junk Eva
Sent: Friday, December 29, 2006 8:21 PM
To: ccielab@groupstudy.com
Subject: How 2 Subnet
Hi,
Can anyone please explain CIDR in detail
can you please explain why the router is showing me this?
R1(config)#int lo 400
R1(config-if)#ip add 0.0.0.1 0.0.0.0
Not a valid host address - 0.0.0.1
R1(config-if)#ip add 1.0.0.1 0.0.0.0
Bad mask /0 for address 1.0.0.1
if
Prefix Length /0
Netmask 0.0.0.0
Inverse Netmask 255.255.255.255
Number of Unique IPs 4,294,967,296
Number of Class A Networks 256
Number of Class B Networks 65,536
Number of Class C Networks 16,777,216
What I'm doing wrong?
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