1. Copyright 1999, S.D. Personick. All Rights Reserved. Telecommunications Networking II Lecture 33 The Domain Name System (DNS) Ref: Tanenbaum 622-630

2. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System (DNS) 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 The concept is very similar to telephone numbers like 1-888-ProfStu

3. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System (DNS) The Domain Name System is based on a hierarchical naming scheme Root govmileducomorgnet us (etc.)

4. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System (DNS) Examples: sdp@ece.drexel.edu sdp2@bellatlantic.net cpersoni@notes.cc.telcordia.com mmaeda@darpa.mil beisenstein@ieee.org rkahn@cnri.reston.va.us wclinton@whitehouse.gov

5. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System (DNS) Domain names are case-insensitive SDP@ECE.Drexel.edu = sdp@ece.drexel.edu but….some Web page names, e.g., WWW.Drexel.edu/… have case-sensitive fields after the “/” that follows the top level domain name (e.g.,.edu,.com,.org,.gov,...)

6. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System Top level domains are either “generic” :.com,.edu,.gov,.mil,.net,.org or “geographic”:.us,.jp,.it, …. The domain naming structure is about to be modified in order to introduce competition into the process of assigning domain names

7. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System Domains are managed by assigned entities.edu is managed by Network Solutions Inc (a publicly traded subsidiary of SAIC Inc), under contract from the U.S. Government (NSF). If you want to establish a new.edu domain name (example: ucphila.edu), you ask Network Solutions Inc to assign this name to you (if available)

8. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System.gov and.mil are also managed by Network Solutions Inc under contract with the U.S. Government (last I looked).jp is managed by an an entity appointed by the japanese government.com is currently managed by Network Solutions Inc, but …..

9. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System …there is a plan being implemented, as we meet, to open up the assignment of domain names to competition There will be registries and registrars; and.com will be supplemented with other commercial and non-commercial top level domain names (e.g.,.firm,.bus,.nom)

10. Copyright 1999, S.D. Personick. All Rights Reserved. The Domain Name System 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 Note: Assignment of names is a very contentious and litigious process

11. Copyright 1999, S.D. Personick. All Rights Reserved. DNS Queries 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 Inc administers.edu -Drexel Univ administers drexel.edu -The Drexel ECE Department can administer ece.drexel.edu

12. Copyright 1999, S.D. Personick. All Rights Reserved. DNS Queries 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. Translations can be cached, but are typically not cached for long (at least not at the lower levels of the hierarchy)

13. Copyright 1999, S.D. Personick. All Rights Reserved. DNS Queries 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

14. Copyright 1999, S.D. Personick. All Rights Reserved. DNS Queries Drexel DNS DARPA DNS.mil DNS Root DNS.edu DNS 1 2 3 IP address of: mmaeda@darpa.mil

15. Copyright 1999, S.D. Personick. All Rights Reserved. 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