Functionality equals vulnerability. Networks connect computers to share information and resources. That is the functionality. Once connected, what is shared is no longer private and protected. That is the vulnerability. An unplugged computer is secure. By contrast, a networked computer is vulnerable. There you have it.
The vulnerabilities, generally speaking, are that shared resources may become unavailable, corrupted, or shared with the wrong people. So we put in the network to facilitate business and then mitigate the risk by maintaining on availability, integrity, and confidentiality.
These three areas are common in all types of networks. Now, the major classifications of networks are Lans, Wans, Mans, and Pans. That is, respectively, Local Area Networks, Wide Area Networks, Metropolitan Area Networks, and Personal Area Networks. All four can be broken. All four can lose data or eat emails. And, of course, all four can leak confidential information.
In fact, at the network level, it is best to assume no confidentiality. Sure, there might be some in Lans. But this can be broken. If you are going out over a Wan link, you have no control over the equipment. There is no guarantee that someone is not reading your data. In fact, according to the news lately, there is a good chance that someone is reading your data. Address confidentiality issues at higher layers, and focus on network availability and integrity.