Protocol:
Protocol is a set of rules which is used by computers to communicate with each other across network.
As there are various ways in which computers communicate with other, there are various protocols as well. Among them few are:

IP (Internet Protocol):

• Used to deliver data packets from source to destination based on their address.

UDP (User Datagram Protocol or Universal Datagram Protocol):

• Connectionless protocol
  (Before sending data, it doesn’t make sure whether the destination system is available or not)
• Does not provide acknowledgement
  (It doesn’t acknowledge, whether the data was successfully sent or not)
• The overall communication is faster comparable to TCP, as it doesn’t spend time to establish connection and for acknowledgement.
• Used primarily for broadcasting audio and video etc over a n/w

TCP (Transmission Control Protocol):

• Connection oriented protocol.
  (Before sending data, it send hello packet to the destination system to make sure whether it is available or not)
• Establish connection between two hosts and exchange stream of data.
• Flow control
  (It  guarantees that packets will be delivered in the same order in which they are sent)
• Error control
  (If there is a damage in packets during the transmission, it resend that packet)
• Provide acknowledgment
  (It provide you acknowledgement that the data was successfully delivered.)

PPP (Point to Point Protocol):

• Used to connect any individual computer or a network to the ISP.

DHCP (Dynamic Host Configuration Protocol):

• Automatically provide IP Address setting to the computers in a network.
• Port no. 67 - for DHCP Server
• Port no. 68 - for DHCP Client

FTP (File Transfer Protocol):

• Used to download files from FTP Server to the client computer and upload file to the FTP server from Client computer.
• Port no. 21 – used by server for incoming connection from client.
  (Connection, response, command passes using this port)
• Port no. 20 – outbound control (data flow)

HTTP (Hyper Text Transfer Protocol):

• Used to fetch the web page from web server.
• Port no. 80

TFTP (Trival File Transfer Protocol):

• It is used to boot workstations, routers etc which did not have any data storage device.
• Port no. – 69

Telnet (Network Virtual Terminal Protocol):

• Used to work remotely on a computer, router and switch etc.
  (You can install and configure telnet in your computer, router and switch etc so that you can work on them from other computers in a network)
• Insecure communication (as data packet flow in plain text (unencrypted), if any hacker captures the data packet they can read it)
• Port no. – 23

SSH (Secure Shell):

• Same as work as Telnet.
• Secure communication (as the packet flow is encrypted)
• Port no. – 22

SMTP (Simple Mail Transfer Protocol):

• Used to send mail to the mail server.
• Port no. 25

IMAP (Internet Message Access Protocol):

• Used to receive mail from the mail server.
• Port no. 143

POP (Post Office Protocol):

• Used to receive mail from the mail server.
• Port no. 109 (POP2)
• Port no. 110 (POP3)

DNS (Domain Name System):

• Used to resolve (translate) domain name to IP address and IP address to domain name.
• Port no. 53

ARP (Address Resolution Protocol):

• Used to resolve IP address to mac-address.

RARP (Reverse Address Resolution Protocol):

• Used to resolve mac-address to IP address.