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A broadcast and/or a Broadcast in a computer network is a message, with which packets will transfer from one point to all participants of a net. In the exchange technique a Broadcast is a special form of the multipoint connection.

A Broadcast is predominantly used in a computer net, if the address of the receiver of the message is still unknown. An example of it are minutes ARP and DHCP. Likewise a Broadcast serves the simple transmission of information to all participants of a net, in order not to have to transmit contrary to the Unicast the same information several times. If information is to be sent only to selected participants in this case, one uses multicast procedures.

Each receiver of a Broadcasts must receive and decide the message whether he must process the message. If the receiver does not recognize itself responsibly as, it tacitly rejects the message.

Network-able computer games use Broadcasts in the local network (not in the Internet, here normally a central server used), in order to find a list of all open plays, in which the user can participate. Of SMB minutes when using Windows likewise by Broadcast one tries to find printer and network releases in the local network.

Broadcasts gives it on different levels of the OSI reference model. All it is common that Broadcasts of a higher level must be adapted on the level of the used physical network. So e.g. a IP-Broadcast in a Ethernet network must as Ethernet Broadcast to the MAC address FF: FF: FF: FF: FF: FF to be dispatched.

If the underlying network is not broadcast able, because it consists e.g. of a quantity of point-to-point connections (e.g. Internet), the nodes must be supplied by means of an Flooding algorithm with the information.

IP-Broadcasts

Different forms are differentiated from IP-Broadcasts:

Limited Broadcast as a goal is indicated the IP address 255.255.255.255. This goal lies always in the own net and directly into a Ethernet Broadcast is converted. Limited a Broadcast not passed on by one rout. Directed Broadcast the goal are the participants of a certain net. The address is indicated by the combination of goal net and setting all host bits to 1. Therefore the address for one reads directed Broadcast into the net 192.168.0.0 with the net mask 255.255.255.0 (192.168.0.0 /24): 192.168.0.255. Directed a Broadcast passed on by one rout, if pouring and goal net are different and only converted in the goal net into a Broadcast. If pouring and goal net are identical, this corresponds to one limited Broadcast. Often this special case is called also local Broadcast. A Directed Broadcast can be further regarded differentiated. The Broadcast can as Subnet directed Broadcast, when universe subnets directed Broadcast appear or as Net directed Broadcast. Subnet directed a Broadcast has as a goal a fixed Subnetz of a network. Universe subnets directed a Broadcast is a Broadcast in all Subnetzen of a network and Net directed a Broadcast in a classified network, which is not divided in Subnetze, is distributed (e.g. Broadcast to the address 10.255.255.255 is distributed in class a A IP network).

Due to safety problems with DOS attacks the preset behavior was changed by routing in RFC 2644 for directed broadcasts. Rout should not directed broadcasts not pass on.

IPv6 supports no more Broadcasts, it instead Multicasts uses.

See also: Unicast, multicast, Anycast

Related links

  • RFC 826 - Ethernet Address Resolution Protocol (English)
  • RFC 1812 - Requirement for IP version 4 Routers (English)
  • RFC 2644 - Changing the default for Directed Broadcasts in Routers (English)

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