A broadcast in a computer network is a message in which data packets are transmitted from one point to all subscribers of a message network. In switching technology, a broadcast is a special form of multipoint connection.
A broadcast packet reaches all subscribers of a local network without them being explicitly specified as recipients. As a result, broadcasts should be limited to their own network segment and not forwarded by routers. If information is to be sent to a group of selected subscribers, a multicast procedure is used instead.
Each recipient of a broadcast decides for himself whether, if he is responsible, he either processes the message received or otherwise tacitly rejects it. Broadcasts exist on different layers of the OSI reference model. What they all have in common is that broadcasts from a higher layer must be adapted accordingly to the lower layers. For example, an IPv4 broadcast is sent as an Ethernet broadcast to the MAC address FF:FF:FF:FF:FF:FF. If the underlying network is not broadcast-capable, for example because it consists of a number of point-to-point connections, the message can instead be sent using a flooding algorithm.
A broadcast is used in a computer network, among other things, if the IP address of the recipient of the message is still unknown. According to the OSI model, this technique is used in the switching layer. Examples are ARP, DHCP and Wake On LAN. Network-capable computer games use broadcasts to find a list of all open games on the local network in which the user can participate. The SMB communication protocol uses broadcast to search for file and printer shares on the local network.
Broadcasts in IPv4 are realized via a group address. A distinction is made between different types of IP broadcasts:
The IP address 255.255.255.255 is specified as the destination. This destination is always located in the own local network and is directly converted into an Ethernet broadcast. A limited broadcast is not forwarded by a router.
The destination are the participants of a specific network. The address is specified by the combination of the target network and the setting of all host bits to 1. The address for a direct broadcast to the network 192.168.0.0 with the netmask 255.255.255.0 is therefore: 192.168.0.255 (CIDR notation: 192.168.0.255/24). A directed broadcast is forwarded by a router if the source and destination networks are different and is only converted into an Ethernet broadcast in the destination network. If the source and destination networks are identical, this corresponds to a limited broadcast. This special case is often referred to as local broadcast. A directed broadcast can be considered further differentiated. The broadcast can occur as subnet-directed broadcast, as all-subnets-directed broadcast or as net-directed broadcast. A subnet-directed broadcast has a fixed subnet as its target. An all-subnets-directed broadcast is a broadcast in all subnets of a network, and a net-directed broadcast is distributed in a classified network that is not divided into subnets (for example, broadcast to the address 10.255.255.255 is distributed in a class A IP network).
Due to security issues with DoS attacks, the default behavior of routers in RFC 2644 for directed broadcasts has been changed. Routers should not forward directed broadcasts.