Virtual LAN

A local area network (LAN) can be thought of as a collection of computers tied together on the same "piece of wire" who share access to, and compete for, the bandwidth provided.  If one user on a LAN accesses the network, then others cannot.  If two machines access the [Ethernet] network at exactly the same time, then their communications collide with each other and cause the network to pause for a brief period of time.  This shared network creates a "collision domain" which consists of all machines which can collide with each other's communications. 

Networks, such as Missouri State University's, are divided into many segments or LANs which are tied together via routers.  These routers divide up the network into smaller parts called subnets each of which is a single collision domain in a LAN environment.

By contrast, a VLAN or Virtual Local Area Network, usually consists of two or more collision domains but is still considered a single subnet.  At the time of this writing (11-11-1998), New Hall consists of a single subnet and a single collision domain and, therefore, is not a VLAN.  Carrington Hall, however, consists of eight collision domains and a single subnet.  Therefore, Carrington Hall's network is a virtual LAN.   Glass Hall consists of four collision domains and four subnets, therefore there are no VLANs in Glass Hall either.

The VLAN is created by connecting multiple collision domains (or even fileservers or workstations) to a device called a switch.  Since the whole VLAN is addressed as a single subnet, the switch must keep track of which MAC Addresses lie in which collision domain.  The switch then sends communications for that machine only out the single port that connects the device.  All other ports, thus collision domains, remain unburdened with extraneous traffic.  All-points, subnet broadcasts, however, are propagated to all collision domains.  This proves to be the limiting factor when determining the number of stations to place on a VLAN.

Since Missouri State University currently runs IP, IPX, and a small amount of AppleTalk, we have chosen to define about 350 stations per VLAN and can grow to about 500 stations per VLAN especially as AppleTalk continues to diminish in use.