Links
[1] https://hootsgo.org/?q=node/6302
[2] http://www.tutorialspoint.com/data_communication_computer_network/network_layer_introduction.htm
[3] http://www.tutorialspoint.com/data_communication_computer_network/network_addressing.htm
[4] http://www.tutorialspoint.com/data_communication_computer_network/network_layer_routing.htm
[5] http://www.tutorialspoint.com/data_communication_computer_network/internetworking.htm
[6] http://www.tutorialspoint.com/data_communication_computer_network/network_layer_protocols.htm