Introduction to Networks 6.0 Instructor Materials – Chapter 10: Application Layer

Instructor Planning Guide


What activities are associated with this chapter?


Students should complete Chapter 10, “Assessment” after completing Chapter 10.

Quizzes, labs, Packet Tracers and other activities can be used to informally assess student progress.

Sections & Objectives

10.1 Application Layer Protocols

Explain the operation of the application layer in providing support to end-user applications.

Explain how the functions of the application layer, session layer, and presentation layer work together to provide network services to end user applications

Explain how common application layer protocols interact with end user applications.

10.2 Well-Known Application Protocols and Services

Explain how well-known TCP/IP application layer protocols operate.

Explain how web and email protocols operate.

Explain how DNS and DHCP operate.

Explain how file transfer protocols operate.

Chapter 10: Application Layer

10.1 – Application Layer Protocols

10.1.1 – Application, Presentation, and Session– Application Layer

Application Layer:

  • Closest to the end user.
  • Used to exchange data between programs running on the source and destination hosts.– Presentation and Session Layer

Presentation Layer function:

  • Formatting data at the source device into a compatible form for the receiving device.
  • Compressing data.
  • Encrypting data.

Session Layer Function

  • Create and maintain dialogs between source and destination applications.– TCP/IP Application Layer Protocols

Post Office Protocol (POP) TCP 110 – Enables clients to retrieve email from a mail server.

Internet Message Access Protocol (IMAP) TCP 143 – Enables clients to retrieve email from a mail server, maintains email on server.

File Transfer Protocol (FTP) TCP 20 and 21 – Reliable, connection-oriented, and acknowledged file delivery protocol.

Trivial File Transfer Protocol (TFTP) UDP 69 – simple connectionless file transfer protocol.

Hypertext Transfer Protocol (HTTP) TCP 80, 8080 – Set of rules for exchanging text, graphic images, etc. on the World Wide Web.

Hypertext Transfer Protocol Secure (HTTPS) TCP, UDP 443 – Uses encryption and authentication to secure communication.

Domain Name Server (DNS) TCP,UDP 53 – Translates domain names, such as, into IP addresses.

(BOOTP) – Bootstrap Protocol – BOOTP is being superseded by DHCP.

Dynamic Host Configuration Protocol (DHCP) UDP client 68, server 67 – Dynamically assigns IP addresses to client stations at start-up.

Simple Mail Transport Protocol (SMTP) TCP 25 – Enables clients to send email to a mail server.

10.1.2 – How Application Protocols Interact with End-User Applications– Client-Server Model

Client and server processes are considered to be in the application layer.

Application layer protocols describe the format of the requests and responses between clients and servers.

Example of a client-server network is using an ISP’s email service to send, receive and store email.– Peer-to-Peer Networks

Data is accessed from a peer device without the use of a dedicated server.

Each device (known as a peer) can function as both a server and a client. – Peer-to-Peer Applications

A P2P application allows a device to act as both a client and a server within the same communication.

P2P applications require that each end device provide a user interface and run a background service. – Common P2P Applications

Common P2P networks include:

  • G2
  • Bitcoin
  • BitTorrent
  • eDonkey

Some P2P applications are based on the Gnutella protocol, where each user shares whole files with other users.

Many P2P applications allow users to share pieces of many files with each other at the same time –this is BitTorrent technology. – Lab – Researching Peer-to-Peer File Sharing

10.2 – Well-Known Application Layer Protocols and Services

10.2.1 – Web and Email Protocols – Hypertext Transfer Protocol and Hypertext Markup Language

When a web address or uniform resource locator (URL) is typed into a web browser, the web browser establishes a connection to the web service running on the server, using the HTTP protocol. – HTTP and HTTPS

HTTP is a request/response protocol.

Three common HTTP message types are:

  • GET – A client request for data.
  • POST – Uploads data files to the web server.
  • PUT – Uploads resources or content to the web server.

HTTP Secure (HTTPS) protocol uses encryption and authentication to secure data. – Email Protocols

Email clients communicate with mail servers to send and receive email.

Mail servers communicate with other mail servers to transport messages from one domain to another.

Three protocols for email:

  • Simple Mail Transfer Protocol (SMTP) to send email.
  • Post Office Protocol (POP) to retrieve email.
  • Internet Message Access Protocol (IMAP) to retrieve email. – SMTP Operation

SMTP is used to send email– POP Operation

POP is used to retrieve email from a mail server.

Email is downloaded from the server to the client and then deleted on the server. – IMAP Operation

IMAP is used to retrieve mail from a mail server.

Copies of messages are downloaded from the server to the client and the original messages are stored on the server. – Packet Tracer – Web and Email

10.2.2 – IP Addressing Services – Domain Name Service

Domain names convert the numeric address into a simple, recognizable name.

The DNS protocol defines an automated service that matches resource names with the required numeric network address. – DNS Message Format

When a client makes a query, the server’s DNS process first looks at its own records to resolve the name.

If unable to resolve, it contacts other servers to resolve the name.

The server temporarily stores the numbered address in the event that the same name is requested again.

The ipconfig /displaydns command displays all of the cached DNS entries on a Windows PC. – DNS Hierarchy – The nslookup Command

Nslookup – a utility that allows a user to manually query the name servers to resolve a given host.

Can also be used to troubleshoot name resolution issues and to verify the current status of the name servers. – Dynamic Host Configuration Protocol

The Dynamic Host Configuration Protocol (DHCP) for IPv4 automates the assignment of IPv4 addresses, subnet masks, gateways, and other parameters.

DHCP-distributed addresses are leased for a set period of time, then returned to pool for reuse.

DHCP is usually employed for end user devices. Static addressing is used for network devices, such as gateways, switches, servers, and printers.

DHCPv6 (DHCP for IPv6) provides similar services for IPv6 clients. – DHCP Operation – Packet Tracer – DHCP and DNS Servers – Lab – Observing DNS Resolution

10.2.3 – File Sharing Services – File Transfer Protocol

FTP requires two connections between the client and the server, one for commands and replies, the other for the actual file transfer:

  • The client establishes the first connection to the server for control traffic using TCP port 21.
  • The client establishes the second connection to the server for the actual data transfer using TCP port 20. – Server Message Block

The Server Message Block (SMB) is a client/server file sharing protocol:

  • SMB file-sharing and print services have become the mainstay of Microsoft networking.
  • Clients establish a long-term connection to servers and can access the resources on the server as if the resource is local to the client host. – Packet Tracer – FTP – Lab – Exploring FTP

10.3 – Summary

10.3.1 – Conclusion – Packet Tracer – Explore a Network – Packet Tracer – Multiuser – Tutorial – Packet Tracer Multiuser – Implement Services – Chapter 10: Application Layer

Explain the operation of the application layer in providing support to end-user applications.

Explain how well-known TCP/IP application layer protocols operate.

New Terms and Commands

Bootstrap Protocol (BOOTP)

Simple Mail Transfer Protocol (SMTP)

Post Office Protocol (POP)

Internet Message Access Protocol (IMAP)

File Transfer Protocol (FTP)

Trivial File Transfer Protocol (TFTP)


Server Message Block (SMB)

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