Topic 21


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Topic 21

  1. 1. Telecommunications Networking II Topic 21 The Domain Name System (DNS) Ref: Tanenbaum 622-630 Dr. Stewart D. Personick Drexel University
  2. 2. The Domain Name System ( DNS ) <ul><li>The principal purpose of the Domain Dame System ( DNS ) is to allow Internet hosts, and other entities that have IP addresses, to refer to each other by “names” that are easier to remember than 32-bit IP addresses </li></ul><ul><li>The concept is very similar to telephone numbers like 1-888-ProfStu </li></ul>
  3. 3. The Domain Name System ( DNS ) <ul><li>The Domain Name System is based on a hierarchical naming scheme </li></ul>Root gov mil edu com org net us (etc.)
  4. 4. The Domain Name System ( DNS ) Examples: [email_address] edu [email_address] net [email_address] com [email_address] mil [email_address] org [email_address] us [email_address] gov
  5. 5. The Domain Name System ( DNS ) Domain names are case-insensitive = but….some Web page names, e.g.,… have case-sensitive fields after the “/” that follows the Top-Level domain name (e.g., .edu, .com, .org, .gov, ...)
  6. 6. The Domain Name System <ul><li>Top-Level domains are either “generic” : .com, .edu, .gov, .mil, .net, .org or “geographic”: .us, .jp, .it, …. </li></ul><ul><li>The original domain naming structure is being modified in order to introduce competition into the process of assigning domain names </li></ul>
  7. 7. <ul><li>Domains are managed by assigned entities (Registries + Registrars) </li></ul><ul><li>.org is managed by Network Solutions® (a subsidiary of Verisign®): historically, under a contract from the U.S. Government. If you want to establish a new .org domain name (example:, you ask Network Solutions to assign this name to you (if available). </li></ul>The Domain Name System
  8. 8. <ul><li>.gov and .mil are managed by an entities under contract with the U.S. Government </li></ul><ul><li>.jp is managed by an an entity appointed by the Japanese government </li></ul><ul><li>The .com “Registry” is currently managed by Network Solutions Inc, but there are now multiple “Registrars” </li></ul><ul><li>New generic Top-Level Domains have been established by ICANN (e.g., .biz, .info, .pro, .aero, .name, .coop, .museum) </li></ul>The Domain Name System
  9. 9. The Domain Name System <ul><li>The US Postal Service has suggested that it assign “.us” Internet names to everyone in the U.S., and that it administer the .us domain </li></ul><ul><li>Note: Assignment of names is a very contentious and litigious process </li></ul>
  10. 10. DNS Queries <ul><li>The naming hierarchy allows for distributed administration of the assignment of domain names…and the tables that link domain names to IP addressses: -Network Solutions administers .edu -Drexel Univ administers -The Drexel ECE Department can administer </li></ul>
  11. 11. <ul><li>Each entity that administers a part of the domain name space must maintain a server at a known address (or more than one server) which can be queried to translate domain names into IP addresses. </li></ul><ul><li>Translations can be cached, but are typically not cached for long (at least not at the lower levels of the hierarchy) </li></ul>DNS Queries
  12. 12. <ul><li>If you need to translate a domain name into an IP address, you ask your (“local”) domain name server. If it doesn’t have the translation ( authoritative or cached), it will send a DNS query to a higher level server and/or another server at its own level in the hierarchy </li></ul>DNS Queries
  13. 13. DNS Queries Drexel DNS DARPA DNS .mil DNS Root DNS .edu DNS 1 2 3 IP address of: [email_address]
  14. 14. DNS Resource Records Domain_name Time_to_live Class Type Value See Tanenbuam p 625: Time_to_live = integer (seconds, e.g., 86400) Class= IN (Internet information) Example Type: A (IP address of a host) Value: 32-bit integer