As stated above, a collision domain is a link where collisions can occur between transmitting devices. For example a hub with several people attached to it or a bus type network connection. Anytime you have more than two devices sharing a common medium (cable, air, etc) there can be collisions. Every port on a switch is a collision domain so an individual pc connected to a switch port has no one to collide with. Broadcasts however, when processed by a switch are flooded out of every port, thus switches are defined as a single broadcast domain. Everyone connected to a switch receives every broadcast generated by any other device connected to that switch. Consider the ramifications if you have 20 or 30 switches nterconnected with one another. A pc on one switch on one side of the network generates a broadcast and that broadcast, even though it may destined to a device right next door, gets flooded to every port on every interconnected switch. Not good. For that reason we put individual ports into VLANS which basically keep traffic seperated by a configured number associated with any specific port. Traffic generated in VLAN1 for example can only be forwarded into VLAN1. There is no interVLAN connectivity unless you put a router in the network and then use it to route between the VLANS. This is whey the devices in one VLAN typically have an IP address in one IP network and devices in other VLANS have addresses associated with a different subnet. VLANS are configured as a layer 3 subnet. The router acts as a border that broadcasts (by default) do not cross. When traffic is generated in one VLAN and needs to go into another VLAN the router simply sees it as subnet to subnet traffic and routes it as normal. If you have further questions please feel free to contact me. Brent Mossberger, bmossberger@gmail.com

As stated above, a collision domain is a link where collisions can occur between transmitting devices. For example a hub with several people attached to it or a bus type network connection. Anytime you have more than two devices sharing a common medium (cable, air, etc) there can be collisions. Every port on a switch is a collision domain so an individual pc connected to a switch port has no one to collide with. Broadcasts however, when processed by a switch are flooded out of every port, thus switches are defined as a single broadcast domain. Everyone connected to a switch receives every broadcast generated by any other device connected to that switch. Consider the ramifications if you have 20 or 30 switches nterconnected with one another. A pc on one switch on one side of the network generates a broadcast and that broadcast, even though it may destined to a device right next door, gets flooded to every port on every interconnected switch. Not good. For that reason we put individual ports into VLANS which basically keep traffic seperated by a configured number associated with any specific port. Traffic generated in VLAN1 for example can only be forwarded into VLAN1. There is no interVLAN connectivity unless you put a router in the network and then use it to route between the VLANS. This is whey the devices in one VLAN typically have an IP address in one IP network and devices in other VLANS have addresses associated with a different subnet. VLANS are configured as a layer 3 subnet. The router acts as a border that broadcasts (by default) do not cross. When traffic is generated in one VLAN and needs to go into another VLAN the router simply sees it as subnet to subnet traffic and routes it as normal. If you have further questions please feel free to contact me. Brent Mossberger, bmossberger@gmail.com