|  | 3.1 | Basic LAN Devices
| 3.1.11 | Network segments

The term segment has many meanings in networking and the correct definition depends upon the situation in which it is used. Historically, a segment identifies the Layer 1 media that is the common path for data transmission in a LAN. As previously mentioned on the media page, there is a maximum length for data transmission on each type of media. Each time an electronic device is used to extend the length or manage data on the media a new segment is created. The devices that are used to create new segments are covered in the rest of the pages of this chapter.
Some people refer to segments by the term wires, though the "wire" might be optical fiber, wireless medium, or copper wire. The function of the different segments of a network are to act as efficient LANs that are part of a larger network.
Other definitions of the term segment are commonly used in networking. Here are two other definitions that will be used in later networking topics. It is important to note that since these topics will be covered later, you may not understand what they mean right now. The only reason that this is discussed here, is to eliminate confusion later when the term segment has a different network meaning.
A second definition, more commonly used by Cisco today, defines a segment as a collision domain. The difference between the first and second definitions is very small and will be defined in a later chapter when collision domains are defined.
Finally, a third definition for segment that you will hear, describes a Layer 4 PDU (Protocol Data Unit). This definition again will be covered in later chapters.