Links
[1] https://hootsgo.org/?q=node/6304
[2] http://www.tutorialspoint.com/data_communication_computer_network/application_layer_introduction.htm
[3] http://www.tutorialspoint.com/data_communication_computer_network/client_server_model.htm
[4] http://www.tutorialspoint.com/data_communication_computer_network/application_protocols.htm
[5] http://www.tutorialspoint.com/data_communication_computer_network/network_services.htm