The Domain Name System (DNS) allows Internet hosts and entities with IP addresses to use easier to remember names instead of numeric IP addresses. DNS uses a hierarchical naming scheme with top-level domains like .com, .edu, and country codes. Domain names are mapped to IP addresses through a system of DNS servers that can look up and cache these mappings.

1. Telecommunications Networking II Topic 21 The Domain Name System (DNS) Ref: Tanenbaum 622-630 Dr. Stewart D. Personick Drexel University

2. 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. The Domain Name System ( DNS ) The Domain Name System is based on a hierarchical naming scheme Root gov mil edu com org net us (etc.)

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. 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. The Domain Name System Top-Level domains are either “generic” : .com, .edu, .gov, .mil, .net, .org or “geographic”: .us, .jp, .it, …. The original domain naming structure is being modified in order to introduce competition into the process of assigning domain names

7. Domains are managed by assigned entities (Registries + Registrars) .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: stampcollectors.org), you ask Network Solutions to assign this name to you (if available). The Domain Name System

8. .gov and .mil are managed by an entities under contract with the U.S. Government .jp is managed by an an entity appointed by the Japanese government The .com “Registry” is currently managed by Network Solutions Inc, but there are now multiple “Registrars” New generic Top-Level Domains have been established by ICANN (e.g., .biz, .info, .pro, .aero, .name, .coop, .museum) The Domain Name System

9. 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

10. 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 administers .edu -Drexel Univ administers drexel.edu -The Drexel ECE Department can administer ece.drexel.edu

11. 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) DNS Queries

12. 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 DNS Queries

13. DNS Queries Drexel DNS DARPA DNS .mil DNS Root DNS .edu DNS 1 2 3 IP address of: [email_address]

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