CIDR II: A little more in depth explanation
PART II:
Suppose our ISP owns an address block: 200.24.0.0/16. What this means?
An address block comprises of different address that the ISP can
allocate and sell to its customers. The IP address block 200.24.0.0/16 can
represent 2^16 = 65, 536 IP addresses. Suppose from this block it wants to
allocate 200.24.15.0/20 address block. So how many addresses is this in
reality? Simple, 2^12 = 4096 or 16 /24s, how? The block size is /20 or in other
words 255.255.240.0 so our block size can be calculated as 256-240 = 16 , this means
the given /20 block comprises of 16 /24 addresses if we are considering a Class
full environment.
200.24.15.0
200.24.16.0
200.24.17.0
200.24.18.0
200.24.19.0
200.24.20.0
200.24.21.0
200.24.22.0
200.24.23.0
200.24.24.0
200.24.25.0
200.24.26.0
200.24.27.0
200.24.28.0
200.24.29.0
200.24.30.0
Each address has the capacity to represent 255 addresses so 255*16 =
4096, as already mentioned above. So if
the ISP distributes these among 3 organizations named A, B, C—the distribution
would be as follows:
200.24.15.0
200.24.16.0
200.24.17.0
Block size of 4, 256-4=252 so the whole block would be 200.24.15.0/30
200.24.18.0 IPblock 200.24.15.0/30 will
be assigned to the Organization A
200.24.19.0
200.24.20.0
200.24.21.0
200.24.22.0
Block size of 8. 256-8 = 248 so the whole block would be 200.24.19.0/21
200.24.23.0 IPblock 200.24.19.0/21
will be assigned to organization B
200.24.24.0
200.24.25.0
200.24.26.0
200.24.27.0
200.24.28.0Block
size of 4, 256-4=252, so the whole block would be 200.24.27.0/30
200.24.29.0 IPblock 200.24.27.0/30 will
be assigned to organization C
200.24.30.0
Believe
me, by doing above process, we have cracked all the route aggregation and
summarization at the Global, ISP and Organization level.
You
will feel that CIDR has the same look like VLSM. Yes, it’s right to some
extent. Both allow us to change the IP dynamics according to our requirements,
but VLSM is invisible to the global internet. The VLSM can be felt only in our
internal network topology. On the other side, CIDR is visible to the global
internet. A global Internet Registry can assign any CIDR block or prefix block
to any top level ISP, to a medium level ISP or to any private organization.
Okay
now some interesting stuff. If you want to see all this CIDR, route
summarization in action, visit MeritRADb the routing asset database online
website. This website provides information related to all the routed networks
and ASs on the internet up to this moment! For example if we want to dig this
IP: 173.194.67.104, we will go to their website: http://www.ra.net/ and will query this IP using Query the RADb box. The output is quite
interesting:
route: 173.194.67.0/24
descr: Google
origin: AS15169
notify: radb-contact@google.com
mnt-by: MAINT-AS15169
changed: radb-contact@google.com 20121119
source: RADB
This
IP (173.194.67.104) is owned by Google incorporations!! Okay one more IP: 205.134.232.114
route: 205.134.224.0/19
descr: Corporate Colocation, Inc.
origin: AS17139
notify: netops@mzima.net
mnt-by: MAINT-CORPCOLO
changed: noc@corporatecolo.com 20071108
source: RADB
The
above IP is somehow part of the major route shown in the RADb output. The
Routing Asset Database website is one of a great place to spend your weekend
time on!
Please
remember that IANA or Internet Assigned
Number Authority is the organization responsible for taking care of global
IP address allocation and other IP related activities.
Categories:
CCNA,
CCNA Notes,
CIDR,
Classless Inter-Domain Routing,
IP Address Conservation
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