The limitations of the host table become obvious when it is used for a large number of hosts. The host table requires every computer to have a local copy of the table, and each copy must be complete because only computers listed in the local host table can be accessed by name.
Consider today's Internet: It has millions of hosts. A host table with millions of entries is very inefficient to search and, more important, is impossible to maintain. Hosts are added to and deleted from the Internet every second. No system could maintain such a large and changeable table and distribute it to every computer on the network.
DNS solves these problems by eliminating the need for an all-inclusive, centrally maintained table by replacing it with a distributed, hierarchical database system. The current DNS database has millions of host entries, distributed among tens of thousands of servers. Distributing the database in this way reduces the size of the database handled by any individual server, which in turn reduces the task of maintaining any individual piece of the database.
Additionally, DNS uses local caching to migrate information close to those who need it, without sending unnecessary information. A caching server starts with just the information it needs to locate the root of the hierarchical database. It then saves all of the answers to user queries that it receives and all of the supporting information learned in gaining those answers. In this way, it builds up an internal database of the information it needs to serve its users.
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