Friday, 26 May 2017

Limited broadcast address and Directed broadcast address

 

Before discussing Limited broadcast address (LBA) and Directed broadcast address (DBA), let's get familiarized with some common terms which will be used in this tutorial.
  • The first term is cast. Cast means to transmit though it has some other popular meaning in networks it means to transmit or send.
  • The second term is network id. It is the part of the IP address that is used to determine the network to which a particular host becomes. For example, if 122.234.78.23 is an arbitrary IP address which belongs to class A. Then 122.0.0.0 is the network id of the network. 
A network id can be defined as a valid network number with all zeroes in the host number part.
  • The third term is host id. It is the part of the IP address that is used to determine a host in a network.
With this knowledge let's start our tutorial on LBA and DBA.

Limited Broadcast Address:

Consider the following diagram it shows a LAN of 6 nodes connected via a switch.


Let's say the node named Jacob wants to send an introduction message to all the other 5 nodes. One of the brute force methods is to send the message one by one to everyone. In this case, he'll have to make 5 packets each having the same data saying "Hello, My name is Jacob". Such type of casting is called as uni-cast where a host sends a packet to single host only. Clearly using uni-cast in this scenario is inefficient. So what should we do?

This is where LBA comes into the picture. If we want to send a packet to ALL the other host in our local network than we use LBA. Remember that using LBA we cannot send a packet to a particular group of people (which is called as multicasting), it is sent to all the hosts in our network.
The packet structure would be something like this.

Data: Hello, My name is Jacob.
Source IP address: Jacob's IP address.
Destination IP address: 255.255.255.255
Source MAC address: Jacob's MAC address.
Destination MAC address: FF:FF:FF:FF:FF:FF
Point to remember here is that whenever we set destination IP as 255.255.255.255 it is always considered as LBA by the end mile router and the packet is never forwarded by the router.

Directed broadcast address:

Now consider the following diagram which shows two networks connected via a router.


Let's say Jacob wants to send a message to all the nodes in a network named csmonkk. Again he has the option of sending the message one by one to all the nodes in the csmonkk network. But it is not an efficient way of networking. This is possible if the type of network is small. But if we want to broadcast it to a class A (every network contains nearly 16 million hosts) network then it becomes nearly impossible to send the packet to everyone.
Using DBA we can achieve this easily. Using DBA if a host in a network A wants to send a packet to all the hosts in some other network say csmonkk (Assume Network id of csmonkk is 122.0.0.0) then we use DBA.

The structure of the packet is something like this:
Data: Hello, My name is Jacob.
Source IP address: Jacob's IP address.
Destination IP address: 122.255.255.255
Source MAC address: Jacob's MAC address.
Destination MAC address: MAC address of the default router.

So if we want to send a packet to all the other host in some other network, then substitute host id part by one.


For example, If IP address is 194.245.9.2 then what is DBA?

We take the IP address and find out to which class this IP address belongs. Clearly, we can see that it belongs to class C network. In class C the first 24 bits represent network number and the rest 8 represent host number. So we take the host part and replace it with all one's.
194.245.9.00000010  ➡ 194.245.9.11111111
      194.245.9.2  ➡  194.245.9.255
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