Chapter 31 - Naming with the Domain Name System

• IP addresses vs. symbolic names

• Structure of Computer names

• DNS naming structure

• DNS server hierarchy

• An example of Name resolution

• Sections 31.8 to 31.16 will not be covered

Introduction• IP assigns 32-bit addresses to hosts (interfaces)

– Binary addresses easy for computers to manage

– All applications use IP addresses through the TCP/IP protocol software

– Difficult for humans to remember:

% telnet 134.82.11.70

• The Domain Name System (DNS) provides translation between symbolic names and IP addresses

• DNS is the naming scheme used in the Internet

Structure of DNS names

• Each name consists of a sequence of alphanumeric components separated by periods

• Examples:

– www.eg.bucknell.edu

– www.netbook.cs.purdue.edu

– charcoal.eg.bucknell.edu

• Names are hierarchical, with most-significant component on the right

• Left-most component is computer name

DNS naming structure• Domain: a part of the computer naming hierarchy used in the Internet.• Eg, commercial organization have names registered under the .com domain• Top level domains (right-most components; also known as TLDs) defined by global authority

DNS naming structure

• Organizations apply for names in a top-level domain:

– bucknell.edu

– macdonalds.com

• Organizations determine own internal structure

– cs.stanford.edu

– cs.purdue.edu

– ie.cuhk.edu.hk

Geographic structure• Top-level domains are US-centric

• Geographic TLDs used for organizations in other countries: TLD Country .ca Canada .uk United Kingdom .fr France .ch Switzerland .hk Hong Kong .cn China .jp Japan .sg Singapore .au Australia

• Countries define their own internal hierarchy: ac.uk and .edu.au are used for academic organizations in the United Kingdom and Australia

Domain names within an organization

• Organizations can create any internal DNS hierarchy • Authority for creating new subdomains is delegated to each domain

• Administrator of foobar.com has authority to create soap.foobar.com and candy.foobar.com without having to contact any central naming authority

Example DNS hierarchy

DNS and client-server computing

• DNS names are managed by a hierarchy of DNS servers

• Hierarchy is related to DNS domain hierarchy

• Root server at top of tree knows about next level servers

• Next level servers, in turn, know about lower level servers

DNS server hierarchy

Root server: authority for the top-level domains

Name Servers• Partition hierarchy into administrative authority responsible for portion of the hierarchy

edu com

princeton … mit

cs ee

ux01 ux04

physics

cisco … yahoo nasa … nsf arpa … navy acm … ieee

gov mil org net uk fr

2 ways of how the organization divides the domain name hierarchy among its 3 DNS servers

Example: sending E-mail

Name

server

Mail

program

User

TCP

IP

2

cs.princeton.edu

192.12.69.5

3

user @ cs.princeton.edu

1

192.12.69.5 4

192.12.69.5 5

Example: sending E-mail

1. User presents domain name of host (eg, cs.princeton.edu) to an application program (eg, a mail program)

2. Application program (mail program in this example) engage the naming system (name server program) to translate the domainname of host into host address (ie IP address).

3. Application (mail program) opens a connection to the host bypresenting its IP address to TCP

Summary

• Domain Name System maps from computer names and IP addresses

• Important to hide 32-bit IP addresses from humans

• DNS names are hierarchical and allocated locally