如果你對整個網路還不熟悉，那麼當你第一次開始瞭解不同型別的地址以及它們是如何協同工作的時候，你會覺得有點不知所措。今天的超級使用者問答旨在為好奇的讀者澄清困惑。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

圖片由維基百科提供。

問題

超級使用者讀者user2449761想了解更多關於乙太網/MAC地址的需求：

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 mechani** 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?

為什麼需要乙太網/MAC地址？

答案

超級使用者貢獻者Paul為我們提供了答案：

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.

請務必透過下面的連結閱讀其他生動的討論主題！

有什麼要補充的解釋嗎？在評論中發出聲音。想從其他精通技術的Stack Exchange使用者那裡瞭解更多答案嗎？在這裡檢視完整的討論主題。