Chapter 15 The Application Layer:

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31436 Systems Software NetworksAUTUMN 2002

• Chapter 15 The Application Layer
• Domain Naming System

UTS Faculty of IT is a Cisco Regional Networking
Academy
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Application Layer Basics

• Layer 7 of the OSI model closest to users.
• Supports the communications components of
  applications.
• Provides a user interface to the lower layers of
  the OSI model
• Direct Web browser, E-mail client, FTP client.
• Indirect Word processor, Spreadsheet, etc.
• When the user wishes to use the network, they
  must do so via the application layer.

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I want to be connected!

• We can find computers on networks using
• IP address
• And just how many IP addresses do you know off by
  heart???
• NetBIOS name (Network Basic I/O System)
• Flat naming scheme on older Windows networks,
  usually used in conjunction with WINS servers.
• Suitable for a small LAN, but not feasible on the
  wider Internet.
• To make things easier, DNS was developed.

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Domain Naming System

• Uses characters and digits to form a recognisable
  domain name instead of using pure numbers.
• Effectively a distributed database to map between
  hostnames and IP addresses.
• Hierarchical naming system.
• Developed for human convenience rather than
  computer efficiency
• WinSock applications connect to other computers
  over TCP/IP using the IP address.

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DNS Hierarchy

• Top of DNS hierarchy is the Root domain 13
  servers globally maintained by ICANN
• Next level down are top-level domains.
• .com, .org, .edu, .gov, .net, .au, .uk, .tw, etc
• Second-level domains
• microsoft.com, whitehouse.gov, edu.au
• Third-level domains
• windowsupdate.microsoft.com, uts.edu.au

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DNS Namespace

• Hierarchical namespace is flexible.
• Many computers can have the same name.
• Provided the hierarchy level is different.
• Therefore, every Fully Qualified Domain Name must
  be unique.
• FQDN comprises the computers host name and its
  domain membership name.

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Fully Qualified Domain Name

• Compare these DNS addresses
• www.uts.edu.au
• www.it.uts.edu.au
• www.eng.uts.edu.au
• All computers have identical
• Hostname www
• Top-level domain au
• Second-level domain edu
• Third-level domain uts
• Difference is the fourth-level domain.

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Show me the website!!!

• Dont recall IP addresses, just the DNS address!
  Behind every DNS address is an IP address.
• Cisco Systems
• www.cisco.com 198.133.219.25
• Microsoft
• www.microsoft.com 207.46.197.100
• Sun Microsystems
• www.sun.com 192.18.97.241
• Benetton Formula 1 Racing Team
• www.benettonf1.com 193.178.121.168

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Before we show that website

• Humans deal with domain names.
• Computers deal with IP addresses.
• A translator is required to resolve the DNS
  hostname to an IP address.
• The answer is DNS hostname resolution.

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DNS Hostname Resolution

• Domain info stored in zone database files.
• Forward Lookup Zone
• Resolves hostnames to IP addresses
• All queries are examined and executed from right
  to left.
• Reverse Lookup Zone
• Resolves IP addresses to hostnames using the
  in-addr.arpa domain.
• Results of all queries to DNS servers are cached
  for later use.

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Welcome to my world )

• Daniel COMARMOND
• E-Mail dacomarm_at_it.uts.edu.au
• ICQ 6606132
• Phone [phone]
• Website http//charlie.it.uts.edu.au/dacomarm
• Take care, and SMILE!!! )