It provide the diagrammatic representation of various services provided by the application layer to the end user or application client.

1. The Application Layer
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2. DNS – The Domain Name System
• The DNS Name Space
• Resource Records
• Name Servers
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3. The DNS Name Space
A portion of the Internet domain name space.
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4. Name Servers
Part of the DNS name space showing the division into zones.
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5. Name Servers
How a resolver looks up a remote name in eight steps.
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6. Electronic Mail
• Architecture and Services
• The User Agent
• Message Formats
• Message Transfer
• Final Delivery
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7. Reading E-mail
An example display of the contents of a mailbox.
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8. Message Formats – RFC 822
RFC 822 header fields related to message transport.
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9. Message Formats – RFC 822
Some fields used in the RFC 822 message header.
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10. MIME – Multipurpose Internet Mail
Extensions
Problems with international languages:
• Languages with accents
(French, German).
• Languages in non-Latin alphabets
(Hebrew, Russian).
• Languages without alphabets
(Chinese, Japanese).
• Messages not containing text at all
(audio or images).
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11. MIME
RFC 822 headers added by MIME.
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12. MIME
The MIME types and subtypes defined in RFC 2045.
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13. Final Delivery
I. Sending and reading mail when the receiver has a permanent Internet
connection and the user agent runs on the same machine as the message
transfer agent.
II. Reading e-mail when the receiver has a dial-up connection to an ISP.
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14. The World Wide Web
• Architectural Overview
• Static Web Documents
• Dynamic Web Documents
• HTTP – The HyperText Transfer Protocol
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15. Architectural Overview
The parts of the Web model.
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16. The Client Side
(a) A browser plug-in. (b) A helper application.
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17. The Server Side
A multithreaded Web server with a front end and processing modules.
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18. The Server Side
A server farm.
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19. The Server Side
(a) Normal request-reply message sequence.
(b) Sequence when TCP handoff is used.
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20. URLs – Uniform Resource
Locaters
Some common URLs.
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21. HTTP Methods
The built-in HTTP request methods.
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22. References
[1] A. S. Tanenbaum, “Computer Network”, Pearson Education
[2] Forouzen, “Data Communication and Networking”, TMH
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