10-06-2017, 06:43 AM
Definition
These are special purpose devices and computers that just transfer messages from one network to another. Before we look deep into the topic Virtual LAN's, let us see the basic devices used in the network backbone. They are
1. Bridges.
2. Switches.
3. Routers.
4. Gateways.
5. Hubs.
BRIDGES :-Bridges operate at the data page link layer. They connect two LAN segments that use the same data page link and network protocol.
SWITCHES :-Like bridges, switches operate at the data page link layer. Switches connect two or more computers or network segments that use the same data page link and network protocol.
ROUTERS :-Routers operate at the network layer. Routers connect two or more LANs that use the same or different data page link protocols, but the same both the basic system interconnection and the necessary translation between the protocols in both directions.
HUBS :- Physical layer devices that are really just multiple port repeaters. When an electronic digital signal is received on a port, the signal is reampli-fied or regenerated and forwarded out all segments except the segment from which the signal was received.
In a broadcast environment, a broadcast is sent out by a host on a single segment would propagate to all segments, saturating the bandwidth of the entire network. Also, without forcing some method of checking at an upper layer, all devices in the broadcast domain would be able to communicate via Layer 2.This severely limits the amount of security that could be enforced on the network. Before the introduction of switches and VLANs, networks were divided into multiple broadcast domains by connectivity through a router .Because routers do not forward broadcasts, each interface is in a different broadcast domain. Each segment is an individual IP subnet and regardless of a workstation's function, its subnet is defined by its physical location.
A group of devices on one or more logically segmented LANs (configured by use of software), enabling devices to communicate as if attached to the same physical medium, when they are actually located on numerous different LAN segments. VLANs are based on logical instead of physical connections and thus are tremendously flexible.
A VLAN is logical broadcast domain that can span multiple physical LAN segments .A VLAN can be designed to provide independent broadcast domains for station logically segmented by functions, project teams, or applications without regard to the physical location of users. Each switch port can only be assigned to only one VLAN. Ports in a VLAN share broadcasts. Ports that do not belong to the same VLAN do not share
broadcasts. This control of broadcast improves the network's overall performance.
VLANs enable switches to create multiple broadcast domains within a switched network. Any user in this VLAN would receive a broadcast from any other member of the VLAN; users of other VLANs would not receive these broadcasts. Each of the users in a given VLAN would also be in the same IP subnet.