Network+ 2008 Tutorial

Addressing Schemes: unicast, multicast, broadcast

There are several ways you can transmit data on a network. As you transmit data, or packets, information travels from the source client to the destination client. There are three ways information can be transmitted: unicast, multicast, and broadcast.

In a situation where you are transmitting information packets from a single source to a single destination, you are transmitting data in a unicast method. If you are able to transmit data from a single source to multiple destinations (think of an Internet radio station), you can save money on bandwidth by multicasting the data.

In an IT environment, if you use Ghost to image several workstations at a time, you are using multicasting to transmit the packets. The single source (the Ghost Server) is transmitting a packet of data to several destinations (the workstations).

The third method of transmitting data is to broadcast it. A broadcast sends the data
from one client to every client on the network. A hub operates as a broadcasting network device – every client on it receives the same packets.

Have you ever heard of a broadcast storm? A broadcast storm occurs when a large accumulation of network broadcasts occurs on a switch. The most common cause of a broadcast storm is when a loop is created on a switch or on connected switches. If you connect an Ethernet cable into two ports on a switch, you can cause a broadcast storm. As broadcasts are sent out, the loop broadcasts, then repeats the broadcast, until it amplifies out of control. Newer switches eliminate this problem by detecting it and disabling those ports.

I have seen broadcast storms take down entire local area networks. You must be careful when you attach switches into a network to ensure you do not accidentally take down a network with a broadcast storm.

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