Describe the purpose and basic operation of the protocols in the OSI and TCP/IP models

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

Every time you switch on your computer, you see a number of applications running on your computer. From the operating system to the small utilities, all are applications.

A networked application is an application that uses two components to run. One of its components runs on the local computer and the other component runs on a network computer or a remote host to provide communication to the host running on the local computer.

The networked applications mostly use Internet and other network hardware to perform their tasks. The web browser is an example of networked application. A networked application uses the application layer protocols such as HTTP, SMTP, and FTP to communicate with servers and other applications.

Many times a protocol and its application is so closely tied with each other that it is difficult to separate them. For example, the applications such as telnet, DHCP, FTP, SNMP are all networked applications that are very closely tied to the protocols they are using.

A web browser, which is also a web application allows you to type a URL in the address box and takes you to the destination page using HTTP or HTTPS protocols.The web browser first connects you to the server referred to in the URL by resolving the DNS name with the IP address of the server. The browser then sends HTTP get request to the server and requests the path information. The server responds to the request with a file that point to other files, images, fonts, and links on some other or the same server. The browser reads the file and opens the desired URL. The HTTP URL changes to HTTPS when a secure website needs to be opened.

The other common networked applications are:


The email application uses SMTP and POP3 protocols to send and receive messages to/from an email server and email client. When you send an email message, the email client that you use sends the message to the email server using SMTP protocol. The email server reads the domain name of the destination email server and then sends the message to that server. If the server with the specified domain does not exist then the message is returned back to the email client.

The message delivered to the email server is spooled or queued on the destination server. The destination server checks the queue and delivers the message to the destination email account. The email client uses POP3 protocol to download the message or receive the email.

Remote login Telnet

The Telnet allows you to connect to a remote computer where you have an account. During a telnet session, your keyboard and mouse seem connected with the remote computer. A telnet client allows terminal emulation by logging into the telnet server. Once connected, telnet allows you to use remote computer resources as your local computer resources. The only problem with a telnet session is that, it sends username and password on the network in plain text.

Network file system (NFS)

NFS allows two different types of file systems to share files and interoperate on different computers on a Local Area Network (LAN). NFS allows data stored on network servers to be accessed by different types of client computers on the LAN. NFS is a distributed file system that is used in both UNIX based systems and other operating systems such as Mac OS, MS Windows, IBM and AS/400. If you want to access UNIX files from a Windows system then NFS allows you to access UNIX files by temporarily storing those files in RAM. Even after having different file naming restrictions, both types of users can access the same files in a normal manner.

File Transfer between two accounts on two computers (FTP)

The file transfer application uses FTP protocol to transfer files between two computers. The FTP uses an FTP client computer and an FTP server to operate. The client initiates the FTP process by accepting the username and password and creates an FTP session. During an FTP session, a client can upload and download files from the FTP server, change directories, and list files. Although you can upload and download files using HTTP protocol also, FTP is typically used for transferring large files and fast throughput.

Domain Name Service (DNS)

DNS resolves Fully Qualified Domain Name (FQDN) or host names to its IP address. It allows you to access websites by typing their user friendly names instead of remembering their IP address and typing it in a browser to access that website. The use of DNS allows you to change the IP address of a website as often as you want because it allows you to use domain names instead of IP address.

Dynamic Host Configuration Protocol (DHCP)

DHCP allows you to assign IP address to the host computers on your network dynamically. In the absence of DHCP, each computer on a network needs to be assigned a unique IP address manually. The manual assignment of IP addresses become a pain when a computer moves to a different network segment in a large network and needs to be assigned a different IP address. In large network environments the dynamic assignment of IP addresses eases the administrator’s task of managing IP addresses.

The DHCP assigns the IP address to a computer as soon as it is switched on. When a DHCP client is switched on it sends a broadcast request (DISCOVER packet) to the DHCP server. The DHCP server determines the appropriate address for the client based on the range of IP addresses available to it and sends back (OFFER packet) to the client.

The DHCP server configures the DNS server, WINS server, and NTP server on the client computer. The client sends a REQUEST packet to the server to let the server know that it has accepted the offered IP address. The DHCP server acknowledges the request by sending the ACK packet.

Example Question

Which of the following applications uses the POP3 protocol to transfer information between a server and a host?

  • E-mail
  • Web browser
  • FTP
  • Telnet

Answer: A

E-mail applications use Post Office Protocol (POP) to download e-mail messages to client computers.

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