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!

The whole site in one document for easy reference!
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 Global Unicast Address Format
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback
Next Topic/Section

IPv6 Interface Identifiers and Physical Address Mapping
(Page 1 of 2)

In IPv4, IP addresses have no relationship to the addresses used for underlying data link layer network technologies. A host that connects to a TCP/IP network using an Ethernet network interface card (NIC) has an Ethernet MAC address and an IP address, but the two numbers are distinct and unrelated in any way. IP addresses are assigned manually by administrators without any regard for the underlying physical address.

The Payoff of IPv6’s Very Large Address Size

With the overhaul of addressing in IPv6, an opportunity presented itself to create a better way of mapping IP unicast addresses and physical network addresses. Implementing this superior mapping technique was one of the reasons why IPv6 addresses were made so large. With 128 total bits, as we saw in the previous topic, even with a full 45 bits reserved for network prefix and 16 bits for site subnet, we are still left with 64 bits to use for the interface identifier, which is analogous to the host ID under IPv4.

Having so many bits at our disposal gives us great flexibility. Instead of using arbitrary “made-up” identifiers for hosts, we can base the interface ID on the underlying data link layer hardware address, as long as that address is no greater than 64 bits in length. Since virtually all devices use layer two addresses of 64 bits or fewer, there is no problem in using those addresses for the interface identifier in IP addresses. This provides an immediate benefit: it makes networks easier to administer, since we don't have to record two arbitrary numbers for each host. The IP address can be derived from the MAC address and the network identifier. It also means we can in the future tell the IP address from the MAC address and vice-versa.

The IPv6 Modified EUI-64 Format

The actual mapping from data link layer addresses to IP interface identifiers depends on the particular technology. It is essential that all devices on the same network use the same mapping technique, of course. By far the most common type of layer 2 addresses in networking are IEEE 802 MAC addresses, used by Ethernet and other IEEE 802 Project networking technologies. As you may already know, these addresses have 48 bits, arranged into two blocks of 24. The upper 24 bits are arranged into a block called the organizationally unique identifier (OUI), with different values assigned to individual organizations; the lower 24 bits are then used for an identifier for each specific device.

The IEEE has also defined a format called the 64-bit extended unique identifier, abbreviated EUI-64. It is similar to the 48-bit MAC format, except that while the OUI remains at 24 bits, the device identifier becomes 40 bits instead of 24. This provides gives each manufacturer 65,536 times as many device addresses within its OUI.

A form of this format, called modified EUI-64, has been adopted for IPv6 interface identifiers. To get the modified EUI-64 interface ID for a device, you simply take the EUI-64 address and change the 7th bit from the left (the “universal/local” or “U/L” bit) from a zero to a one.


Previous Topic/Section
IPv6 Global Unicast Address Format
Previous Page
Pages in Current Topic/Section
1
2
Next Page
IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback
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.