Why are Ethernet/MAC Addresses Needed?

If you are still new to the whole networking thing, then it can be a bit overwhelming when you first start learning about the different kinds of addresses involved and how they work together. Today’s SuperUser Q&A post seeks to clear up the confusion for a curious reader.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

Image courtesy of Wikipedia.

The Question

SuperUser reader user2449761 wants to know more about the need for Ethernet/MAC addresses:

I do not understand why Ethernet/MAC addresses are needed. Surely all computers could just be connected to a unified network and use IP addresses to communicate?

For example, there is the following mechanism in Ethernet:

• A computer with the IP address 192.168.1.1 (X.1) wants to send a packet to the address 192.168.1.2 (X.2).
• X.1 uses ARP to get the MAC address of X.2.
• To do so, X.1 needs to send a packet to all computers in the network and only one will answer.
• X.1 gets a MAC address and sends the packet.

It would be simpler to just do it in one step:

• X.1 sends a packet to all computers in the network and only X.2 will process it, the others will ignore it.

My other question is: Why are IP addresses needed if all devices have unique MAC addresses?

Why is there a need for Ethernet/MAC addresses?

The Answer

SuperUser contributor Paul has the answer for us:

The different network layers are there to allow them to be swapped for different technologies. The two layers you are talking about here are Layers 2 and 3. Layer 2 in this scenario is Ethernet – from which MAC addresses arise, and Layer 3 is IP.

Ethernet only works at the local level between network devices connected to a broadcast network “data link”, whereas IP is a routable protocol and can target devices on remote networks.

The requirements for each of these layers is different. Ethernet specifies a family of technologies that allow packets to be sent and received between network devices, whereas IP defines a protocol that allows packets of data to traverse multiple networks.

Neither is reliant on the other, which is what gives networking its flexibility. For example, you may choose to connect to your Internet service using IP over Ethernet, but in your internal network, you might choose to use IP over paper (where someone writes down the contents of each packet and physically walks it over to another machine and types it in). Clearly this would not be particularly fast, but it would still be IP provided the person carrying around the bits of paper respected IP routing rules.

In the real world there are different data link protocols that you are already using (although their addressing schemes are the same): 802.3 – Ethernet, and 802.11 – Wi-Fi.

IP does not care what the underlying layer is. Equally, IP can be swapped out for different network layer protocols (provided it happens for all participants) such as Asynchronous Transfer Mode (ATM).

While there is nothing directly preventing the creation of a protocol that encompasses both Layers 2 and 3, it would be less flexible, less attractive, and therefore unlikely to be used.

Make sure to read through the rest of the lively discussion thread via the link below!

---

Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.