In the previous article we have talked about how the network hub broadcasts all incoming communications to all connected computers (except the sender, of course). This guarantees that the intended destination will get the message, but how does it know the message is intended for it?
To solve this, when the sender machine builds up the data packet, it will indicate the destination by encoding the destination identity into the packet. That way, a receiving computer may extract the destination identity from the packet it receives, compares the extracted identity to its own identity, and if it matches, it means the packet is intended for that receiving computer.
But what is the identity to use?
In the networking world, there are multiple identities that can be used - each has its purpose and pros/cons.
The most basic one is the hardware identity of the network adapter embedded by
the manufacturer - called the MAC address (Machine Access Control address).
This identity is guaranteed to be universally unique for each network adapter/
port.
So, if a sender knows the MAC address of its destination, it can encode it into its data packet, and when the receiver gets the message, it will compare it to its own MAC address, and then will process the data accordingly. The other machines on the network that get the same message will just ignore it because it is not meant for them.
On a machine running Windows, you can find the MAC address of the network card
on your computer by running the below command and looking for the
Physical Address in the output:
ipconfig /all
...
Ethernet adapter vEthernet (Private):
...
Physical Address. . . . . . . . . : 00-11-22-33-44-55
...On a machine running Azure Linux, you can achieve the same by running the below
command and looking for the link/ether in the output:
ip a
...
link/ether 00:11:22:33:44:55
...Of course, there is a security risk here. If a rogue machine managed to be connected to that network, it can listen to all traffic taking place.
The security risk is not the only problem! Let's say you have 100 machines in the network, and all machines are using the network heavily, how much unnecessary data is being sent to every single one of them? Roughly speaking, about 99% of the traffic it sees is not meant for it. That is a significant waste of resources!
Let's see how this can be improved in our next article.
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