Is one more secure than the other, is so identify them?
Intranet is more secure than the internet as well as the extranet. Intranet allows only members of the organization to access relevant information through passwords and usernames. The encryption and use of other authorization methods to access information make it secure. The internet security depends on the person using the device connected to the internet; extranet also uses a firewall to encrypt data, although it is owned by multi-organizations depending on the contract making it more secure than others.
Can they all be linked together? If, so how.
Intranet extranet and internet can be linked together through the use of internet connections.
Can they be linked to other networks such as cellular, radio frequencies, RFID, and others?
Yes, they can be linked to radio frequencies and cellular networks because they use IP/TCP networks.
What is their primary use? Internet, Intranet, and Extranet.
Internet primary use is to share different information with different people throughout the world. The intranet is used to share confidential and sensitive information with the members of the organization. Extranet is a means of relaying information between organizational members and the external members.
In summary, the internet, extranet, and intranet, although sounding the same, they have a different purpose. Any person does not control the internet, and anybody can access the various information they want. The organization manages intranet, and only members of the firm are allowed to access to the confidential and sensitive information. Extranet is controlled by multi agencies depending based on the contract, and external members can access the information.