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!

Enjoy The TCP/IP Guide? Get the complete PDF!
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 Network Interface / Internet "Layer Connection" Protocols
           9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)
                9  Address Resolution Concepts and Issues

Previous Topic/Section
The Need For Address Resolution
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
Dynamic Address Resolution
Next Topic/Section

Address Resolution Through Direct Mapping
(Page 2 of 3)

Direct Mapping Not Possible With Large Hardware Addresses

Unfortunately, direct mapping only works when it is possible to express the data link layer address as a function of the network layer address. Consider instead the same IP address, 222.101.33.29, running on an Ethernet network. Here, the data link layer addresses are “hard-wired” into the hardware itself (they can sometimes be overridden but usually this is not done). More importantly, the MAC address is 48 bits wide, not 8. This means the layer two address is bigger than the layer three address, and there is no way to do direct mapping, as Figure 46 illustrates.


Figure 46: Address Resolution Problems With Large Hardware Address Size

When the layer two address is larger in size than the layer three address, it is not possible to define a mapping between them that can be used for address resolution.

 


Note: In the case where the hardware address size exceeds the network layer address size, we could do a “partial mapping”. For example, we could use the IP address to get part of the MAC address and hope we don't have duplication in the bits we didn't use. This method is not well-suited to regular transmissions, but is in fact used for resolving multicast addresses in IPv4 to Ethernet addresses.


In general, then, direct mapping is not possible when the layer three address is smaller than the layer two address. Consider that Ethernet is the most popular technology at layer two and uses a 48-bit address, and IP is the most popular technology at layer three and uses a 32-bit address. This is one reason why direct mapping is a technique that is not only not widely used, but that most people don't know about!


Previous Topic/Section
The Need For Address Resolution
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
Dynamic Address Resolution
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.