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 6 (IPv6) / IP Next Generation (IPng)
                     9  IPv6 Addressing

Previous Topic/Section
IPv6 Address and Address Notation and Prefix Representation
Previous Page
Pages in Current Topic/Section
1
23
Next Page
IPv6 Global Unicast Address Format
Next Topic/Section

IPv6 Address Space Allocation
(Page 1 of 3)

After dealing for so many years with the very small IPv4 address space, the enormous number of addresses in IPv6 must have made the IETF engineers feel like kids in a candy shop. They were good kids, however, and didn't run wild grabbing all the candy they could find and gobbling it up. They very carefully considered how to divide the address space for various uses. Of course, when you have this much candy, sharing becomes pretty easy.

The Initial IPv6 Address Space Allocation Plan and the Format Prefix (FP)

As was the case with IPv4, the two primary concerns in deciding how to divide the IPv6 address space were address assignment and routing. The designers of IPv6 wanted to structure the address space to make allocation of addresses to ISPs, organizations and individuals as easy as possible.

At first, this perhaps ironically, this led the creators of IPv6 back full circle to the use of specific bit sequences to identify different types of addresses, just like the old “classful” addressing scheme. The address type was indicated by a set of bits at the start of the address, called the format prefix (FP). The format prefix was conceptually identical to the 1 to 4 bits used in IPv4 “classful” addressing to denote address classes, but was variable in length, ranging from three to ten bits. Format prefixes were described in RFC 2373.

In the years following the publication of RFC 2373, the gurus who run the Internet had a change of heart regarding how address blocks should be considered. They still wanted to divide up the IPv6 address space into variably-sized blocks for different purposes. However, they realize that many people (including myself) were starting to consider the use of format prefixes to be equivalent to the old class-oriented IPv4 system.

Their main concern was that implementors might program into IPv6 hardware logic to make routing decisions based only on the first few bits of the address. This was specifically not how IPv6 is supposed to work; for one thing, the allocations are subject to change. Thus, one of the modifications made in RFC 3513 was to change the language regarding IPv6 address allocations, and specifically, to remove the term “format prefix” from the standard.


Previous Topic/Section
IPv6 Address and Address Notation and Prefix Representation
Previous Page
Pages in Current Topic/Section
1
23
Next Page
IPv6 Global Unicast Address Format
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.