Here's why it's considered a proper noun:
* It's a specific brand or product name: Just like you wouldn't say "I used a computer" but instead specify "I used a Dell" or "I used a Mac," Ethernet is a specific type of networking technology.
* It's capitalized: Proper nouns are typically capitalized, and Ethernet is always capitalized.
* It's unique and not interchangeable: You wouldn't say "I used a network" when you meant "I used Ethernet." Ethernet is unique and distinct from other networking technologies.
Think of it this way:
* Ethernet is like a brand name. It's a proprietary technology developed by a specific company (Xerox) and later standardized by an organization (IEEE).
* "Network" is like a generic term. It refers to any system that connects computers together.
So, just like you wouldn't say "I drove a car" when you meant "I drove a Ford," you wouldn't say "I used a network" when you meant "I used Ethernet."