virtual LAN

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Virtual LAN

Software defined groups of host on a local area network (LAN) that communicate as if they were on the same wire, even though they are physically on different LAN segments throughout a site. To define a virtual LAN, the network administrator uses a virtual LAN management utility to establish membersip rules that determine which hostss are in a specific virtual LAN. Many models may exist but two seem to dominate:

(1) Vitual Segment (or Port-Group) Virtual LAN. These are switched at the data link layer (OSI layer 2). Virtual segments turn an arbitrary number of physical segments into a single virtual segment that funtions as a self-contained traffic domain.

(2) Virtual Subnet Virtual LAN: These are switched at the Network Layer (OSI layer 3). Subnet-oriented virtual LANs are based on subnet addresses used by IP, IPX, and other network layer protocols to normally identify physical networks. Administrators assign one subnet address to a number of switch ports (which may be on different switches and over a backbone). Once identified as a virtual subnet, the selected LANs function as a bridge group - traffic is bridged at Layer 2 within the virtual subnet and routed at Layer 3 between virtual subnets.

virtual LAN

Also called a "VLAN," it is a logical subgroup within a local area network that is created via software rather than manually moving cables in the wiring closet. It combines user stations and network devices into a single unit regardless of the physical LAN segment they are attached to and allows traffic to flow more efficiently within populations of mutual interest.

VLANs are implemented in port switching hubs and LAN switches and generally offer proprietary solutions. VLANs reduce the time it takes to implement moves, adds and changes.

VLANs function at layer 2. Since their purpose is to isolate traffic within the VLAN, in order to bridge from one VLAN to another, a router is required. The router works at the higher layer 3 network protocol, which requires that network layer segments are identified and coordinated with the VLANs. This is a complicated job, and VLANs tend to break down as networks expand and more routers are encountered. The industry is working towards "virtual routing" solutions, which allows the network manager to view the entire network as a single routed entity. See 802.1q.

Virtual LANs solve the problem of containing traffic within workgroups that are geographically dispersed. They allow moves, adds and changes to be performed via software at a console rather than manually changing cables in the wiring closet.
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