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)

Overview and History of TCP/IP Host Names and Name Systems 1 2 3 4 TCP/IP Domain Name System (DNS)

Host Table Name Resolution

Name resolution in this system is very simple. Each device reads the host table into memory when it starts up. Users of the system can make reference to the names in that host table in their invocation of various applications, by using a name instead of a numeric IP address. When the software detects a name has been used in this manner, it refers the name to the internal resolver routine in the device, which looks up the name in the host table in memory and returns its address. There is no need for any transmissions or servers to be contacted; resolution is entirely local.

Now, here is the part where I am supposed to say that name registration in the host table name system is simple as well, right? Well, yes and no. From a purely technical standpoint it certainly is: a name is “registered” on a particular device when the name and corresponding IP address is entered into the device's host table, and that's it.

However, name registration is much more complicated from an administrative standpoint, and this is in fact where we find the major weakness of using host tables. Each network device maintains its own host table independent of the others, usually stored as a file on its local hard disk. This is in contrast to database registration systems where the data is centrally stored and managed. This leads to two very important concerns:

• Consistency: Since every device has its own host table, how do we ensure that information is consistent between all the tables on the different devices?
  
  
• Modifications: How do we ensure that information about new device mappings, or changes to existing ones, is propagated to all devices?

The original mechanism for this was simply hand-editing, with administrators consulting updated published lists of device names. This was a very inefficient method that was prone to error and slow to acknowledge changes to the network. The revised system used a centrally-managed “master” file that was downloaded by all sites on a regular basis. Name registration in this method required that the name/address mapping be submitted to the authority managing the central file, the Network Information Center (NIC).

Overview and History of TCP/IP Host Names and Name Systems 1 2 3 4 TCP/IP Domain Name System (DNS)