What is Broadcasting in Computer Network?

In computer network, there are various ways in which we can transmit data from one node to another node, one node to multiple node, and many node to many nodes. For each type of transmission, there's a term associated with it.  Let's see their examples in brief :

• For node to node: 'Unicast' also known as POINT-TO-POINT communication. Example: cell phone conversation.
• When multiple POINT-TO-POINT communications are made its called - 'Multicast'. The message is received by only those hosts who choose to be a part of a multicast group.  Example: Video conferencing, sending a message targeting number of people at at a time.
• One Node to ALL Nodes - Broadcasting. Today we'll discuss broadcasting in depth.

What is Broadcasting?

Broadcasting in computer networks is a type of communication mechanism that allows the message to be received by all the nodes of a network. The term broadcast in general refers to the transmission of signals from radio or televisions.

Every broadcasted signal is stopped at layer-3 network layer of OSI or to be more practical - at the router. A more technical example of Broadcasting would be: The Address-Resolution-Protocol request (ARP-Request) whenever a host needs to resolve an IP address to its corresponding MAC address it will broadcast a signal asking "Who this IP address belongs to?" and this broadcasted signal is received by every single node in a network domain and then an appropriate node will respond accordingly.

Let's take example of a broadcast message with an IPv4 address, see the below two diagram of two different networks: (All the nodes are represented by green circles in the below diagrams)                                                                  

Network 'A' in the above image has an IPv4 address - 90.0.0.0. This is the network address, now if any host has to send a message that is to be received by all the networks within this network then that host will use the address - 255.255.255.255

Network 'B' in the above image has an IPv4 address - 92.0.0.0. This is the network address now if any host has to send a message that is to be received by all the networks within this network then that host will also use the address - 255.255.255.255

But if any node of Network - 'A' has to broadcast a message to Network - 'B' (ALL the nodes in Network - 'B') then they'll use the broadcast address of a network - 'B' which is - 92.255.255.255. And if the same is to be done from Network - 'B' to Network - 'A' then the broadcast address would be - 90.255.255.255

Key Points on Broadcasting

• Data is sent to all the nodes/stations in the network domain.
• A special broadcast address exist for every network which is used to receive a broadcasted message.
• Not every device want to receive the broadcasted message.
• It generates the most network traffic because the broadcasted message is sent to every node in the network.
• It is less secure. A sensitive message shouldn't be sent to everyone and hence it should be kept in mind before broadcasting a message.
• Examples : Address Resolution Protocol (ARP) requests, Dynamic Host Configuration Protocol (DHCP) requests.
   

Advantages of Broadcasting

• Ease of communication: The main advantage of broadcasting is it allows the message to be delivered to every node in a network without establishing a POINT-TO-POINT communication channel which saves a lot of time & process overhead.
• Network Troubleshooting: One can easily identify the corrupt resource or potential security threats in a large network by sending a broadcast message.
• Reduced Network Traffic: Unlike unicast, which separately sends a message to each node in a network one by one resulting in high volume of traffic, and multicasting which also requires additional network infrastructure and can be complex to manage. But broadcasting doesn't require any additional network infrastructure and can simultaneously send data to all the nodes in a network without increasing traffic.

Disadvantages of Broadcasting

• Broadcast Storm: At any point of time if all of a sudden, a number of devices in a network choose to respond to a broadcasted message by sending another broadcast simultaneously then it would lead to higher network traffic and ultimately result in network failure.
• Limited Control & Security issue: When broadcasting a message, we have limited control i.e the data is sent to everyone. So any message before being broadcasted should be made sure that its intended user is everyone in the network.
• Broadcasting doesn't support large amount of devices, it also doesn't allow the message to be customized based on personal preferences.

Types of Broadcasting 

There are 3 types of broadcasting mainly : 

1. Limited Broadcast

It is a simplest broadcasting in which a message is transmitted to every node within a same network (the destination node exists within the same network), the scope of broadcast doesn't go beyond the local network. And an example IPv4 address would be - 255.255.255.255, and IPv6 : FF02::1

2. Directed Broadcast 

In directed Broadcast, the message is sent to a specific group of hosts within a network that can be identified by their subnets. And the host bits of the destination IPv4 address are all set to 1. Example: 192.168.1.255  

3. Subnet Broadcast

Subnetting refers to creating a smaller network within a larger network that shares a common network address. Whenever a message is to be broadcasted to a specific subnet within a larger network then its called subnet Broadcast. In this, the message is sent to the broadcast address of a specific subnet and the broadcast address of a subnet is the highest possible address. For example if the subnet address is : 192.168.1.0 & the subnet mask is 255.255.255.0 then the broadcast address would be 192.169.1.255.

How Broadcasting Works?

It works in following steps :

1. The sender of the message to be broadcasted will initiate the process by setting the destination address as the broadcast address of the network.
2. The message is encapsulated into the network frame with the destination MAC Address set to the broadcast MAC Address.
3. The message is then finally broadcasted, and is received by all the devices of the intended network and the devices that receive this message check if the broadcast matches with their own IP Address.
4. If its a match, then device proceeds with the message further else it discards it.

How To Calculate Broadcast Address?

Case 1: 

Consider an IP address of class A -  10.20.15.3 and the default subnet mask for class A is 255.0.0.0. In each octet, there are 8 bits. So now at bit-level we'll perform an OR operation on 'inverted' Subnet Mask. See the below diagram :

Final Broadcast Address : 10.255.255.255

Case 2:

Consider an IP address of class C -  192.168.3.33 and the subnet mask given is: 255.255.255.248.

Perform same Bitwise OR operation on the given IP & subnet mask till 3^rd octet.

But in the 4^th octet as we can see its value is neither 0 nor 255 so we'll have to take some ADDITIONAL STEPS : 

• There is a total of 256 values for each octet (0-255). So we'll subtract 248 from 256. 256-248 = 8.
• 8 here is called the multiplier. Now we have to determine how many multiples of 8 we need to come up with in order to obtain a value just GREATER than - 33 (4th octet in the given IP - 192.168.3.33).
• Checking multipliers of 8 (keep adding 8 until you get a number greater than 33) :-
  • 8 + 8 = 16      (16 < 33)
  • 16 + 8 = 24     (24 < 33)
  • 24 + 8 = 32     (32 < 33)   
  • 32 + 8 = 40     (40 > 33)
•  We got our answer - 40 which is immediate greater than 33 and is the multiplier of 8.
•  Now, the next step is to subtract 1 from 40. 40-1 = 39. And put 39 in the last octet of our broadcast address and that's the final broadcast address : 192.168.3.39 
   

Difference between Broadcasting and Multicasting

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• Computer Networks-Network Layer