Connecting Devices and Virtual LANs

Introduction
Hosts or LANs do not normally operate in isolation. They are connected to one another or to the Internet. To connect hosts or LANs, we use connecting devices. Connecting devices can operate in different layers of the Internet model. After discussing some connecting devices, we show how they are used to create virtual local area networks (VLANs). The chapter is divided into two sections. The first section discusses connecting devices. It first describes hubs and their features. The section then discusses link-layer switches (or simply switches, as they are called), and shows how they can create loops if they connect LANs with broad- cast domains. The second section discusses virtual LANs or VLANs. The section first shows how membership in a VLAN can be defined. The section then discusses the VLAN configuration. It next shows how switches can communicate in a VLAN. Finally, the section mentions the advantages of a VLAN.
Connecting Devices
Hosts and networks do not normally operate in isolation. We use connecting devices to connect hosts together to make a network or to connect networks together to make an internet. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: hubs, link-layer switches, and routers. Hubs today operate in the first layer of the Internet model. Link-layer switches operate in the first two layers. Routers operate in the first three layers (See Figure1).