The internetworking environment is governed by two complementary rule sets: standards and models. Standards are the laws that vendors must adhere to if they are to interoperate with other vendors, in turn making themselves available and useful for the end user. Some vendors develop special features that can be configured and used only on their equipment; these are called proprietary features. Keep in mind, a proprietary implementation can limit itself in its use and therefore is not always an attractive option when implementing a network.
The OSI model is the universal model in the networking environment and is made up of seven layers. Each of the seven layers provides services to the layer above it and depends on the layer below. The seven layers of the OSI model from top to bottom are (7) application, (6) presentation, (5) session, (4) transport, (3) network, (2) data link, and (1) physical.
The application, presentation, and session layers are known as the upper layers; the transport, network, data link, and physical layers are known as the lower layers.
The OSI model uses encapsulation and decapsulation, depending on where data is moving through the model. The sending side wraps, or encapsulates, the data, much like enclosing a letter in an envelope. The receiving side unwraps, or decapsulates, the data, much like opening an envelope and removing the contents.
Another internetworking model used is the Cisco Hierarchical Design Model, made up of three layers: core, distribution, and access. The core layer provides for high-speed connectivity in a network backbone and is the most efficient and direct path between two points. The distribution layer provides for policy routing. This layer controls the answers to network questions such as these: "How can I get there from here?" and "Who can I allow to go there?" The access layer controls access to the network, keeping local connectivity out of the network, such as a local communication between a computer and a network printer that does not need to go across the WAN.
Numerous network standards are in place today, and many new standards are being developed all the time. The three primary standards bodies to note are the ITU-T, ANSI, and the IEEE. The ITU-T (International Telecommunication Union?Telecommunication Standardization Sector), as the name implies, is the international standards body and can be found on the World Wide Web at www.itu.int/ITU-T/. ANSI (American National Standards Institute), as its name also implies, is the governing standards body for North America and can be found on the World Wide Web at www.ansi.org. The IEEE (Institute of Electrical and Electronics Engineers) is a leading technical authority and is responsible for standards across several technology-oriented fields, such as computer and network engineering, and is regarded as the second-leading international standards body, after the ITU-T. The IEEE can be found on the World Wide Web at www.ieee.org.