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To operate efficiently, the infrastructure of a network consists of devices that conform to well-known standards and protocols. A protocol provides a precise sequence of steps that each element of a network must follow to enable communications. Protocols also define the precise format of all data that is exchanged in a network. For example, the Internet Protocol (IP) defines the format of IP addresses: four eight-bit numbers called octets whose decimal values range from 0 to 255, as in 10.0.101.155.

A standard is a detailed definition of a protocol that has been established by a standards organization and that vendors follow when they create products. Without standards, it would be impossible for one vendor’s products to work with another vendor’s. Because of standards, you can instead purchase equipment from different vendors with the assurance that they’ll work together.

Network standards are organized into a framework called the Open Systems Interconnection (OSI) Reference Model. The OSI Reference Model establishes a hierarchy for protocols so that each protocol can deal with just one part of the overall task of data communications. The OSI Reference Model identifies seven distinct layers at which a protocol may operate:

 Physical (layer 1): Describes the mechanical and electrical details of network components such as cables, connectors, and network interfaces.

 Data link (layer 2): Describes the basic techniques that networks use to uniquely identify devices on the network (typically via a MAC address) and the means for one device to send information over the physical layer to another device, in the form of data packets. Switches operate at the data link layer, which means that they manage the efficient transmission of data packets from one device to another.

 Network (layer 3): Handles the routing of data across networks. Routers operate at the network layer.

 Transport (layer 4): Provides for reliable delivery of packets.

 Session (layer 5): Establishes sessions between network applications.

 Presentation (layer 6): Converts data so that systems that use different data formats can exchange information.

 Application (layer 7): Allows applications to request network services.

Although the upper layers of the OSI model (layers 4 through 7) are equally important, in this chapter and the next, I focus on the first three layers of the OSI model — physical, data link, and network. These layers are the ones where the most common types of networking hardware such as cables, interfaces, switches, and routers operate.

Although many different network protocols and standards can be used in various layers of the OSI model, the most common standard found at layers 1 and 2 is Ethernet. Similarly, the most common standard at layer 3 is IP. I cover more about Ethernet and IP in Chapters 2 and 3 of Book 2, but keep in mind that most of what follows in this chapter is related to Ethernet and IP.