Why do computers need both MAC Addresses and IP Addresses?

We know from previous posts that each computer has both an IP Address and a MAC Address assigned to it.

But, why is this?

If every computer in order to communicate with each other needed to be physically connected to each other, all networks either would become complicated very quickly or they would stay small.

Imagine if in order to go to google.com you had to have a physical wire connecting your computer out to Palo Alto California, just to see their website.  Now imagine, you also needed a physical wire to Seattle to connect to msn.com  And now imagine that everyone in the world would need a physical wire just to see a website.

As you can see, it would physically become very complicated very quickly and the internet would have stayed very small.

To get around this physical limitation, they separated computer communication to also contain a logical component, the IP Address.

What we mean by logical is that IP Addresses are routeable.

What this really means is that the entire internet can play a large game of 6 degrees of separation.

Imagine a situation.  Computers A, B, C, and Computers 1,2,3.

Computers A, B, and C know each other.

And Computers 1,2, and 3 know each other.

But Computer C, and Computer 3 are special in that they also know each other. You can see in the image below.

So, if Computer A wants to send a message to Computer 2, how does it do this?  Computer A will send a message to Computer C and ask it to send this message to Computer 2.  Because Computer C doesn’t know Computer 2 directly, it can ask Computer 3 to send the message to Computer 2.

And through this simple message passing, all of the computers A,B,C,1,2, and 3 can talk to each other even though only Computer C and Computer 3 know each other.

Now, not every computer can send to multiple groups of people, but the ones that can are known as ROUTERS.  They are called this because they route messages between different groups of computers.

The Wireless Router you probably have at home does this job exactly.  It is usually physically connected to the public internet through your cable modem, and then also physically connected to the machines you have at home through blue Cat5 cables or wireless communication technology.  It then allows the machines you have physically connected at home to also logically connect to computers on the internet.

Again, this is simplified, but is easier to understand.

So again, IP Addresses are logical and routeable addresses.  Computer A could potentially learn the IP Address of Computer 2.   However, MAC Addresses are physical and are NOT routeable.  So, Computer A could not really learn the MAC Address of Computer 2.

And that’s why computers have both MAC Addresses and IP Addresses.  MAC Addresses handle the physical connection from computer to computer while IP Addresses handle the logical routeable connection from both computer to computer AND network to network.