Before the invention of computers, information security was for the most part a physical security problem. Before examining how security is applied to networks, a basic understanding of network organization is required. In order to reduce the complexity of networks, most are organized in a series of layers. The most commonly encountered layered model is the 7-layer International Standards Organization (ISO) Open Systems Interconnection (OSI) Reference Model. Network security is still aimed at meeting the objectives of providing confidentiality, integrity, availability, and access for legitimate users of the network resources. The Trusted Network Interpretation addresses nine security services split evenly among three categories. The categories are Communications Integrity, Denial of Service, and Compromise Protection. There are two views that can be taken when discussing interconnection of automated information systems: Component Connection View, and Global Connection View. A distributed system is a combination of hardware and software components connected via a network.