CCNA 1 v6.0 Study Material – Chapter 10: Application Layer

Chapter 10 – Sections & Objectives

  • 10.0 Introduction
  • 10.1 Application Layer Protocols
    • 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 layer Protocols and Services
    • Explain how web and email protocols operate.
    • Explain how the IP addressing protocols operate.
    • Explain how file transfer protocols operate.
  • 10.3 Summary

10.1 Application Layer Protocols

Application, Presentation, Session

    • Application Layer
      • Closest to the end user.
      • Application layer protocols help exchange data between programs running on the source and destination hosts.
      • The TCP/IP application layer performs the functions of the upper three layers of the OSI model.
      • Common application layer protocols include: HTTP, FTP, TFTP, DNS.
    • Presentation and Session Layer
      • Format data, compress and encrypt data
      • Common standards for video include QuickTime and Motion Picture Experts Group (MPEG).
      • Common graphic image formats are: GIF, JPEG and PNG
      • The session layer creates and maintains dialogs between source and destination applications.
      • The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle.
    • TCP/IP Application Layer Protocols
      • TCP/IP application protocols specify the format and control information necessary for common Internet functions.
      • Application layer protocols must be implemented in both the source and destination devices.
      • Application layer protocols implemented on the source and destination host must be compatible to allow communication.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 9

How Application Protocols Interact with End-User Applications

    • Client-Server Model
      • Clients request information while servers provide it.
      • Client and server processes are considered to be in the application layer.
      • The contents of the data exchange will depend of the application in use.
      • Email is an example of a Client-Server interaction.
    • Peer-to-Peer Networks
      • Data is accessed without the use of a dedicated server.
      • Two or more computers can be connected to a P2P network to share resources.
      • Every connected end device (a peer) can function as both a server and a client.
      • The roles of client and server are set on a per-request basis.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 10

  • Peer-to-Peer Applications
    • Some P2P applications use a hybrid system, where resource sharing is decentralized.
    • Indexes that point to resource locations are stored in a centralized directory.
    • In a hybrid system, each peer accesses an index server to get the location of a resource stored on another peer.
  • Common P2P Applications
    • Common P2P networks include: eDonkey, G2, BitTorrent.
    • Many P2P applications allow users to share pieces of many files with each other at the same time.
    • A small torrent file contains information about the location of other users and tracker computers.
    • Trackers are computers keeping track of the files hosted by users.
    • This technology is called BitTorrent. There are many BitTorrent clients, including BitTorrent, uTorrent, Frostwire, and qBittorrent.

10.2 Well-Known Application Layer Protocols and Services

Web and Email Protocols

    • Hypertext transfer Protocol and Hypertext Markup Language
      • A URL is a reference to a web server.
      • URLs and URIs are the names most people associate with web addresses.
      • URLs contain the protocol, the server name and the requested filename.
      • Using DNS, the server name portion of the URL is then translated to the associated IP address before the server can be contacted.
    • HTTP and HTTPS
      • The browser sends a GET request to the server’s IP address and asks for the index.html file.
      • The server sends the requested file to the client.
      • The index.html was specified in the URL and contains the HTML code for this web page.
      • The browser processes the HTML code and formats the page for the browser window based on the code in the file.
      • HTTP Is not secure. Messages can be intercepted.
      • HTTPS uses authentication and encryption to secure data.
    • Email Protocols
      • Email is a store-and-forward method of sending, storing, and retrieving electronic messages.
      • Email messages are stored on mail servers.
      • 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.
      • Email relies on three separate protocols for operation: SMTP,POP and IMAP.
    • SMTP Operation
      • SMTP message formats require a message header and body.
      • The header must have a properly formatted recipient email address and a sender address.
      • An SMTP client sends an email by connecting to a SMTP server on port 25.
      • The server receives the message and stores it in a local mailbox or relays the message to another mail server.
      • Users use email clients to retrieve messages stored on the server.
    • POP Operation
      • Messages are downloaded from the server to the client.
      • Email clients direct their POP requests to mail servers on port TCP 110.
      • POP allows for email messages to be downloaded to the client’s device (computer or phone) and removed from the server.
      • A downloaded message resides on the device that triggered the download.
    • IMAP Protocols
      • IMAP is another protocol used to retrieve email messages.
      • Allows for messages to be displayed to the user rather than downloaded.
      • The original messages reside on the server until manually deleted by the user.
      • Users view copies of the messages in their email client software.
      • Support folder hierarchy to organize and store mail.
      • When a user decides to delete a message, the server synchronizes that action and deletes the message from the server.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 11

IP Addressing Services

    • Domain Name Service
      • IP addresses are not easy to memorize.
      • Domain names make server addresses more user-friendly.
      • Computers still need the actual numeric address before they can communicate.
      • The DNS protocol allows for the dynamic translation of a domain name into the associated IP address.
    • DNS Message Format
      • Common DNS records are A, NS, AAAA and MX.
      • DNS servers search its own records first, relaying the client’s request to other servers if it can’t resolve the request.
      • The response is then forwarded to the client.
      • The client often stores previous name resolutions. Use the ipconfig /displaydns to list cached DNS entries on Windows.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 12

    • DNS Hierarchy
      • The DNS protocol uses a hierarchical system.
      • The naming structure is broken down into small, manageable zones.
      • Each DNS server is only responsible for managing name-to-IP mappings for a small portion of the DNS structure.
      • Requests for zones not stored in a specific DNS server are forwarded to other servers for translation.
      • Top-level domains represent either the type of domain or the country of origin. Examples of top-level domains are .com, .org, .au and .co
    • The nslookup Command
      • Use nslookup to place DNS queries.
      • Useful for DNS troubleshooting.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 13

    • Dynamic Host Configuration Protocol
      • Computers need network IP information to communicate over a network.
      • IP information include host and gateway addresses, mask, and DNS server.
      • DHCP allows for automated and scalable distribution of IP information.
      • DHCP-distributed addresses are leased for a set period of time.
      • Addresses are returned to the pool for reuse when no longer in use.
      • DHCP supports IPv4 and DHCPv6 supports IPv6.
    • DHCP Operation
      • The client broadcasts a DHCPDISCOVER.
      • A DHCP server replies with DHCPOFFER.
      • The client sends a DHCPREQUEST message to the server it wants to use (in case of multiple offers).
      • A client may also request an address previously been allocated by the server.
      • The server returns a DHCPACK to confirm the lease has been finalized.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 14

File Sharing Services

    • File Transfer Protocol
      • FTP was developed to allow the transfer of files over the network.
      • An FTP client is an application that runs on a client computer used to push and pull data from an FTP server.
      • FTP requires two connections between the client and the server: one connection for commands and replies and another connection for the actual file transfer.
      • The client initiates and establishes the first connection to the server for control traffic on TCP port 21.
      • The client then establishes the second connection to the server for the actual data transfer on TCP port 20.
      • The client can download (pull) data from the server or upload (push) data to the server.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 15

    • Server Message Block
      • SMB is a client/server file sharing protocol.
      • All SMB messages share a common format.
      • SMB file-sharing and print services have become the mainstay of Windows networking.
      • Microsoft products now support TCP/IP protocols to directly support SMB resource sharing.
      • After the connection is established, the user of the client can access the resources on the server as if the resource is local to the client host.
      • The Mac, LINUX, and UNIX operating systems have their own implementation of SMB.

CCNA 1 v6.0 Study Material - Chapter 10: Application Layer 16

10.3 Summary


  • Explain the operation of the application layer in providing support to end-user applications.
  • Explain how well-known TCP/IP application layer protocols and services operate.

Chapter 10 New Terms and Commands

  • Hypertext Transfer Protocol (HTTP)
  • File Transfer Protocol (FTP)
  • Trivial File Transfer Protocol (TFTP)
  • Internet Message Access Protocol (IMAP)
  • Domain Name System (DNS)
  • Simple Mail Transport Protocol (SMTP)
  • Post Office Protocol (POP)
  • Dynamic Host Configuration Protocol (DHCP)

  • QuickTime and Motion Picture Experts Group (MPEG)
  • Graphics Interchange Format (GIF)
  • Joint Photographic Experts Group (JPEG)
  • Portable Network Graphics (PNG)
  • Client-server model
  • Peer-to-peer network (P2P)
  • P2P networks
  • P2P applications

  • Gnutella protocol
  • BitTorrent
  • Uniform Resource Locator (URL)
  • Uniform Resource Identifier (URIs)
  • Get
  • Post
  • Put
  • HTTP Secure (HTTPS)
  • Secure Socket Layer (SSL)
  • Simple Mail Transfer Protocol (SMTP)
  • Post Office Protocol (POP)

Chapter 10 New Terms and Commands

  • IMAP
  • Port 25 (SMTP)
  • TCP Port 110 (POP)
  • Domain name
  • DNS Protocol
  • Record types: A, NS, AAAA, MX
  • ipconfig /displaydns
  • Top-level domains are: .com, .org, .au, .co
  • Nslookup

  • Dynamic Host Configuration Protocol (DHCP) for IPv4
  • DHCPv6
  • DHCP Discover
  • DHCP Offer
  • DHCP Request
  • DHCP Acknowledgement
  • DHCP Negative Acknowledgement

  • FTP daemon (FTPd)
  • Server Message Block (SMB)

Download Slide PowerPoint (pptx):

Notify of

Inline Feedbacks
View all comments