Ethernet operates at layer 2 of the Open Systems Interconnect, or OSI, model. In other words, it relies on an address unique to the hardware installed on each network adapter, instead of a routable, transferable IP address. Ethernet is more efficient in smaller network environments, but large groups of PCs connected via Ethernet -- including the Internet itself -- benefit from segmentation. There are many ways to segment an Ethernet network.Collision Domains
Setting up collision domains is the first step to segmenting an Ethernet network. Collision domains relate to the Ethernet concept of collision detection: when two PCs on a wire communicate simultaneously, the doubled voltage creates a “backoff” in the two PCs: a randomly generated amount of time which they wait until attempting to transmit again. Large amounts of PCs on a collision domain creates more backoffs. Too many backoffs can result in significant outages. Collision domains are created by upgrading simple Ethernet hubs to switches. Switches create individual collision domains within each port.
Broadcast Domains
Broadcast domains represent the next logical step in Ethernet segmentation. Segmenting a network into separate broadcast domains limits the impact of broadcast. A broadcast is a layer 2 packet designed to go to every PC on the network. Services such as Dynamic Host Control Protocol, or DHCP, rely on broadcasts. DHCP helps a computer that has just connected to a network to automatically receive a network address. Excessive broadcasts, called “broadcast storms,” can prevent other computers in the broadcast domain from communicating. Routers do not forward broadcasts. Two networks connected through a router will be separated into broadcast domains. When one domain gets affected by a broadcast storm, it will not affect the other.
Virtual Local Area Networks
Virtual Local Area Networks use switches to segment broadcast domains regardless of hardware or physical location. In essence, a single VLAN-compatible switch could house hundreds of separate broadcast domains. This configuration is particularly useful in instances where investment in router hardware does not make economic sense. A single connection between the VLAN switch and the router can be “trunked,” meaning all VLANs are passed along to the router interface. The single, physical router interface can then be configured to have multiple subinterfaces that redirect back to the individual VLANs. Some companies even make “layer 3” switches, with the built-in capability to route between VLANs. Multiple switches, from different building floors, could be interconnected, but its Ethernet connections would remain divided according to department or division. These VLANs communicate across any number of the VLAN-compatible switches without the need for a router.
Quality of Service
Quality of Service segments Ethernet connections according to the type of data being sent. This is commonly used on networks that have a Voice over Internet Protocol, or VoIP, phone network running on the same network as computers. Because voice is more prone to delays in transmission, QoS ensures that voice packets gets delivered ahead of regular data packets.
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