Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Searchable, convenient, complete TCP/IP information.
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing
                          9  IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"

Previous Topic/Section
IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
IP Classless Addressing Block Sizes and "Classful" Network Equivalents
Next Topic/Section

IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation
(Page 3 of 4)

"Supernetting": Subnetting the Internet

In theory, then, what CIDR does is provide the central address-assignment authority with the flexibility to hand out address blocks of different sizes to organizations based on their need. However, when CIDR was developed, a shift was made in the method by which public IP addresses were assigned. Having everyone in the world attempt to get addresses from one organization wasn't the best method. It was necessary under the “classful” scheme because the hierarchy was only two levels deep: IANA handed out network IDs to everyone, who then assigned host IDs (or subnetted).

Under CIDR we have many hierarchical levels: we split big blocks into smaller blocks and then still-smaller blocks, and so on. It makes sense to manage blocks in a similar hierarchical manner as well. So, what happens is that IANA/ICANN divides addresses into large blocks, which it distributes to the four regional Internet registries (RIRs): APNIC, ARIN, LACNIC and RIPE NCC. These then further divide the address blocks and distribute them to lower-level national Internet registries (NIRs), local Internet registries (LIRs) and/or individual organizations such as Internet Service Providers (ISPs). This is all explained in the background topic on Internet authorities and registries.

ISPs can then divide these blocks into smaller ones that they allocate to their customers. These customers are sometimes smaller ISPs themselves, which repeat the process. They split their blocks into pieces of different sizes and allocate them to their customers, some of whom are even smaller ISPs and some of whom are “end users”. The number of times this can occur is limited only by how many addresses are in the original block.

It's also worth noting that while CIDR is based on subnetting concepts, subnetting itself is not used in CIDR—or at least, not in the way it is used under “classful” addressing. There is no explicit subnetting using a subnet ID within CIDR: all IP addresses are interpreted only as having a network ID and a host ID. An organization does the equivalent of subnetting by dividing its own network into subnetworks using the same general method that ISPs do. This probably seems a bit confusing. Later in this section I have provided a detailed example of how hierarchical address block assignment and splitting works under CIDR.


Previous Topic/Section
IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages
Previous Page
Pages in Current Topic/Section
12
3
4
Next Page
IP Classless Addressing Block Sizes and "Classful" Network Equivalents
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.