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 TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  Name Systems and TCP/IP Name Registration and Name Resolution            9  TCP/IP Name Systems: Host Tables and Domain Name System (DNS)                 9  TCP/IP Domain Name System (DNS)                      9  DNS Name Servers and Name Resolution                           9  DNS Resolution Concepts and Resolver Operations

DNS Name Resolution Efficiency Improvements: Caching and Local Resolution 1 2 3 DNS Reverse Name Resolution Using the IN-ADDR.ARPA Domain

Changes to Resolution to Handle Special Cases

This example is highly simplified, and also only shows one possible way that servers might be set up. For one thing, it is possible that even though “compsci.googleplex.edu” is in a separate zone from “googleplex.edu”, they might use the same server. In that case, one iteration in the process would be skipped. The example also above doesn't show what happens if an error occurs in the process.

If the domain name entered was an alias, indicated by a CNAME record, this would change the processing as well. CNAME records are used to allow a “constant” name for a device to be presented to the outside world while allowing the actual device that corresponds to the name to vary inside the organization. When a CNAME is used, it changes the name resolution process by adding an extra step: first we resolve the alias to the canonical name and then resolve the canonical name.

For example, Web servers are almost always named starting with “www.”, so at XYZ Industries we want people to be able to find our Web site at “www.xyzindustries.com”. However, the Web server may in fact be shared with other services on “bigserver.xyzindustries.com”. We can set up a CNAME record to point “www.xyzindustries.com” to “bigserver.xyzindustries.com”. Resolution of “www” will result in a CNAME pointing to “bigserver”, which is then itself resolved. If in the future our business grows and we decide to upgrade our Web service to run on “biggerserver.xyzindustries.com”, we just change the CNAME record and users are unaffected.

DNS Name Resolution Efficiency Improvements: Caching and Local Resolution 1 2 3 DNS Reverse Name Resolution Using the IN-ADDR.ARPA Domain