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Protocols in Application Layer

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The application layer is present at the top of the OSI model. It is the layer through which users interact. It provides services to the user. Application layer performs several kinds of functions which are requirement in any kind of application or communication process.

Application Layer Protocol in Computer Network


Telnet stands for the TELetype NETwork. It helps in terminal emulation. It allows Telnet clients to access the resources of the Telnet server. It is used for managing files on the internet. It is used for the initial setup of devices like switches. The telnet command is a command that uses the Telnet protocol to communicate with a remote device or system. Port number of telnet is 23. 


telnet [\\RemoteServer]
: Specifies the name of the server 
to which you want to connect

2. FTP

FTP stands for File Transfer Protocol. It is the protocol that actually lets us transfer files. It can facilitate this between any two machines using it. But FTP is not just a protocol but it is also a program.FTP promotes sharing of files via remote computers with reliable and efficient data transfer. The Port number for FTP is 20 for data and 21 for control. 


ftp machinename


The Trivial File Transfer Protocol (TFTP) is the stripped-down, stock version of FTP, but it’s the protocol of choice if you know exactly what you want and where to find it. It’s a technology for transferring files between network devices and is a simplified version of FTP. The Port number for TFTP is 69.


tftp [ options... ] [host [port]] [-c command] 

4. NFS

It stands for a Network File System. It allows remote hosts to mount file systems over a network and interact with those file systems as though they are mounted locally. This enables system administrators to consolidate resources onto centralized servers on the network. The Port number for NFS is 2049.


service nfs start


It stands for Simple Mail Transfer Protocol. It is a part of the TCP/IP protocol. Using a process called “store and forward,” SMTP moves your email on and across networks. It works closely with something called the Mail Transfer Agent (MTA) to send your communication to the right computer and email inbox. The Port number for SMTP is 25. 



6. LPD

It stands for Line Printer Daemon. It is designed for printer sharing. It is the part that receives and processes the request. A “daemon” is a server or agent. The Port number for LPD is 515. 


lpd [ -d ] [ -l ] [ -D DebugOutputFile]

7. X window

It defines a protocol for the writing of graphical user interface–based client/server applications. The idea is to allow a program, called a client, to run on one computer. It is primarily used in networks of interconnected mainframes. Port number for X window starts from 6000 and increases by 1 for each server.


Run xdm in runlevel 5 


It stands for Simple Network Management Protocol. It gathers data by polling the devices on the network from a management station at fixed or random intervals, requiring them to disclose certain information. It is a way that servers can share information about their current state, and also a channel through which an administrate can modify pre-defined values. The Port number of SNMP is 161(TCP) and 162(UDP). 


snmpget -mALL -v1 -cpublic snmp_agent_Ip_address sysName.0

9. DNS

It stands for Domain Name System. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name might translate to 
The Port number for DNS is 53. 


ipconfig /flushdns

10. DHCP

It stands for Dynamic Host Configuration Protocol (DHCP). It gives IP addresses to hosts. There is a lot of information a DHCP server can provide to a host when the host is registering for an IP address with the DHCP server. Port number for DHCP is 67, 68. 


clear ip dhcp binding {address | * }


HTTP stands for Hypertext Transfer Protocol and HTTPS is the more secured version of HTTP, that’s why HTTPS stands for Hypertext Transfer Protocol Secure. This protocol is used to access data from the World Wide Web. The Hypertext is the well-organized documentation system that is used to link pages in the text document.

  • HTTP is based on the client-server model.
  • It uses TCP for establishing connections.
  • HTTP is a stateless protocol, which means the server doesn’t maintain any information about the previous request from the client.
  • HTTP uses port number 80 for establishing the connection.

12. POP

POP stands for Post Office Protocol and the latest version is known as POP3 (Post Office Protocol version 3). This is a simple protocol used by User agents for message retrieval from mail servers. 

  • POP protocol work with Port number 110.
  • It uses TCP for establishing connections.

POP works in dual mode- Delete mode, Keep Mode.

In Delete mode, it deletes the message from the mail server once they are downloaded to the local system. 

In Keep mode, it doesn’t delete the message from the mail server and also facilitates the users to access the mails later from the mail server.

13. IRC

IRC stands for Internet Relay Chat. It is a text-based instant messaging/chatting system. IRC is used for group or one-to-one communication. It also supports file, media, data sharing within the chat. It works upon the client-server model. Where users connect to IRC server or IRC network via some web/ standalone application program. 

  • It uses TCP or TLS for connection establishment.
  • It makes use of port number 6667.

14. MIME

MIME stands for Multipurpose Internet Mail Extension. This protocol is designed to extend the capabilities of the existing Internet email protocol like SMTP. MIME allows non-ASCII data to be sent via SMTP. It allows users to send/receive various kinds of files over the Internet like audio, video, programs, etc. MIME is not a standalone protocol it works in collaboration with other protocols to extend their capabilities. 


Last Updated : 10 Jun, 2023
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