A group of network devices, such as workstations and servers, connected to each other at the same PHYSICAL location are known as a LAN (Local Area Network). On a such network all hosts connected to a standard Layer 2 switch are in the same broadcast domain. A VLAN (Virtual Local Area Network) is a LOGICAL group of network devices, such as workstations and servers that might be on separate physical networks, but share the same broadcast domain.
A VLAN allows a network of computers and users to communicate in a simulated environment as if they exist in a single physical LAN and are sharing a single broadcast domain. The purpose of implementing a VLAN is to improve the performance of a network or apply security features. Higher-end switches allow the functionality and implementation of VLANs.
There are two types of packets on a VLAN, tagged and untagged. An untagged packet is a regular packet like exists on a regular network. The decision of which VLAN an untagged packet belongs to is determined by the switch. A switch can be configured to assign specific ports to specific VLANs.
If a switch configured to allow tagged packets, and the port which receives the packet is configured to allow tagged packets, it knows which ports it can send the packet to. A switch can also be configured to transmit tagged packets, this allows a VLAN to span more than one switch.
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