What is a broadcast address and how does it work?

The main meaning of the term “broadcast” is sound or video transmissions via the radio network; the secondary meaning describes a form of communication which is sent out in a computer network as a message without specific recipients. What exactly is broadcasting in network technology, how does it work and what part does the broadcast address play here?

Broadcasting – what is it?

Broadcasting in a computer network is transmitting a message which does not require a response to all users of the network.One computer in a network sends a data packet to all other users at the same time. The sender does not need to indicate recipient addresses – this is how the broadcast process differs from unicast, where only a single known recipient is addressed. The general advantage of broadcasting is that information can be distributed without having to be transmitted multiple times.

A special address is required to carry out the procedure, which replaces the recipient addresses in question. This broadcast IP is of particular use if the addresses of the individual network users are not known.

Definition

A broadcast is a multipoint connection in a computer network. A data packet is transmitted from one point to all users of a messaging network in this way. This occurs with the use of the broadcast address.

The sender initiates the broadcast connection and provides the address at which the recipients can contact them. A broadcast works in a similar way to a mailing list: the recipients are not visible to each other and the sender has no way of knowing the addresses of the network users. Only if the users contact the sender one-to-one do they disclose their own address.

Note

Besides the broadcast and unicast procedures, there are also other forms of network communication, such as the multicast procedure, where selected user (groups) are addressed, and the geocast procedure, where this selection is geographically restricted.

Principles of broadcast technology

Addressing is used to transmit data for communication in a network via the Internet protocol, “IP” for short, from one system to another. The IP address enables the clear forwarding and sending of data packets from the source to the destination. As with a telephone number, this address can be divided up into area code and telephone number – but for IP addresses, this is known as the network part and host part. This division occurs with the help of the subnet mask, which is placed over the IP address like a template. A subnet mask is the same length as an IPv4 address. It describes which bit positions work inside the IP address as the network part or host part.

The data and information are sent to all devices on a network via the broadcast address. The network components are responsible for the receipt and processing of the data. The role of the broadcast IP address is to connect all devices with one another on a network.

IP address: an Internet protocol address (IP address) is a numeric description. Each device that is connected to a computer network based on Internet technology is allocated an IP address. This IP address uses the Internet protocol for communication. An IP address fulfills two main tasks: host or network interface identification and location addressing.

Fact

The Internet protocol version 4 (IPv4) defines an IP address as a 32-bit number. Due to the increasing need for and using up of available IPv4 addresses, a new IP version (IPv6) with 128 bits for the IP address was developed in 1995. It became a standard in December 1998. In July 2017, a final definition of the protocol was published. The IPv6 application has been running since the middle of the 2000s.

Ethernet: Software and hardware are connected with one another using Ethernet technology for cable-connected data networks. An exchange of data between locally connected users is possible using this technology. Wireless networks, and in particular company networks and home networks, most commonly use Ethernet. Furthermore, the Ethernet often forms the basis of the Intranet.

MAC (media access control) address: the MAC address is the device address for a network device. It is usually written as a hexadecimal number, for example, 08-00-20-ae-fd-7e or 08:00:20:ae:fd:7e (or: 080020aefd7e).

How does broadcasting work?

A broadcast packet is sent to all users of a local network at once. They do not have to be explicitly named as recipients. The users of a network can open the data packets and then interpret the information, carry out the instructions or discard it.

There is a differentiation between limited broadcast and directed broadcast:

Limited broadcast

With limited broadcast, an IP address is given as a destination. This IP address is always 255.255.255.255. Technically, this broadcast should be sent to all the IP addresses that exist. However, it actually serves as an address for the broadcast within the network. This destination is always in its own network and can therefore be implemented in an Ethernet broadcast. A router does not forward such a packet.

Directed broadcast

With directed broadcast, all recipients are always addressed within the target network. A combination of the number of the target network and the setting of all host bits to 1 produces the broadcast address in this case. If the destination is not located in its own (sub-) network, a router forwards the data packet.

Host bits are the part of an IP address identifying a specific host in a subnet. The subnet mask determines what proportion of the address is used for network bits and for host bits. For example, an IPv4 address 192.168.0.64/26 has a 6-bit host part because 26 of 32 bits are reserved for the network part.

The broadcast address

Each network or subnet has a dedicated broadcast address, through which all users of the network can broadcast.

In a broadcast address, all the host bits are set to the binary value 1, so if all host bits are set to the value 0, this is the subnet address.

Example: IPv4 address 192.128.64.7/24

192.128.64.7 is the IP address and 24 is the subnet mask. The /24 corresponds to the subnet mask 255.255.255.0. The IP address consists of 4 decimals – called octets – which are separated by points. One octet contains 8 bits, which is why IPv4 is a 32-bit address. Each octet can represent a number between 0 and 255. In this case, the whole of the last octet consists of host bits. Therefore, in this example, the broadcast address would be 192.128.64.255 – so all host bits at 1.

This can be read from this chart:

192.128.64.1 = first host address

192.128.64.254 = last host address

192.128.64.255 = broadcast address

Where can you find the broadcast address? The IP address is a 4-digit series of numbers with values from 0 to 255. A broadcast IP address is only assigned once in each network. It is always the last IP address of the subnet.

Note

The network address and the broadcast address are not used as computer IP addresses. In a network, the first address field is reserved for the network and the last for the broadcast address. In the field between these two are the host addresses of the network.

To identify the broadcast address of your network, enter cmd under Windows in your start menu and press the enter key. This opens the Windows console.

Now enter ipconfig /all there to find out all details of your LAN. This command allows you to obtain your own IP address on the intranet, i.e. in your home network, as well as the subnet mask of the network, besides a great deal of other interesting information. You can derive the broadcast IP from these details. On the public Internet, the router works with another IP address allocated by the provider, which you can read in the router interface, for example, or obtain via tools such as ifconfig.me.

In our example with the IP address 192.168.64.172 and the subnet mask 255.255.252.0, we obtain the network address via subnetting: 192.168.64.0. The network mask (in CIDR notation /22) enables 1,022 hosts: 210 (as we have ten zero positions in the subnet masks, which can be either 0 or 1), excluding the network address itself and the broadcast address. This is therefore at the last possible position: 192.168.67.255 – the last ten positions of the IP address stand at 1.


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