A network switch is a device that operates at the Data Link layer of the OSI model—Layer 2. It takes in packets being sent by devices that are connected to its physical ports and sends them out again, but only through the ports that lead to the devices the packets are intended to reach. They can also operate at the network layer--Layer 3 where routing occurs. Switches are a common component of networks based on ethernet, Fibre Channel, Asynchronous Transfer Mode (ATM), and InfiniBand, among others. In general, though, most switches today use ethernet.
Switches vary in size, depending on how many devices you need to connect in a specific area, as well as the type of network speed/bandwidth required for those devices. In a small office or home office, a four- or eight-port switch usually suffices, but for larger deployments you generally see switches up to 128 ports. The form factor of a smaller switch is an appliance that you can fit on a desktop, but switches are also rack-mountable for placement in a wiring closet or data center or server farm. Sizes of rack-mountable switches range from 1U to 4U, but larger ones area also available. Switches also vary in the network speed they offer, ranging from Fast ethernet (10/100 Mbps), Gigabit ethernet (10/100/1000 Mbps), 10 Gigabit (10/100/1000/10000 Mbps) and even 40/100 Gbps speeds. Which speed to choose depends on the throughput needed for the tasks being supported.
Switches also differ in their capabilities. Here are three types.
Unmanaged switches are the most basic, offering fixed configuration. They are generally plug-and-play, which means they have few if any options for the user to choose from. They may have default settings for features such as quality of service, but they cannot be changed. The upside is that unmanaged switches are relatively inexpensive, but their lack of features make them unsuitable for most enterprise uses.
Managed switches offer more functionality and features for IT professionals and are the type most likely seen in business or enterprise settings. Managed switches have command-line interfaces (CLI) to configure them. They support simple network management protocol (SNMP) agents that provide information that can be used to troubleshoot network problems.
They can also support virtual LANs, quality of service settings and IP routing. The security is also better, protecting all types of traffic that they handle. Because of their advanced features, managed switches cost much more than unmanaged switches.
Smart or intelligent switches are managed switches that have some features beyond what an unmanaged switch offers, but fewer than a managed switch. So they are more sophisticated than unmanaged switches, but they are also less expensive than a fully manageable switch. They generally lack support for telnet access and have Web GUIs rather than CLIs. Other options, such as VLANs, may not have as many features as those supported by fully managed switches. But because they are less expensive, they may be a good fit for smaller networks with fewer financial resources and those with fewer feature needs.