Hello,
in general, broadcasts on a LAN are needed for address resolution. When a node needs to communicate with all of the network, it sends a datagram to MAC address 0xFFFFFFFF (a broadcast), which is an address to which the network interface card (NIC) of every host must respond. An IP workstation broadcasts an Address Resolution Protocol (ARP) request every time it needs to locate a new IP address on the network.
For example, the command:
telnet www.cisco.com
translates into an IP address through a Domain Name System (DNS) search, and then an ARP request is broadcast to find the actual station.
You might find this document interesting:
Broadcasts in Switched LAN Internetworks
http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd20e.htm#26934
HTH,
Georg
