Ethereum Name Service (ENS) is a distributed, open, and extensible naming framework built on the Ethereum blockchain.

ENS’s purpose is to connect easy-to-read names like ‘alice.eth’ with computer-friendly identifiers such as Ethereum addresses, other cryptocurrency addresses, content hashes, and metadata. It also enables ‘reverse resolution’, so metadata like canonical names or interface descriptions can be linked to Ethereum addresses.

ENS shares high-level objectives with DNS, the Internet’s Domain Name Service, but its design is quite different to fit what the Ethereum blockchain allows and constrains. Like DNS, ENS uses dot-separated, hierarchical domain names, where the domain owner maintains authority over their subdomains.

Top-level domains such as ‘.eth’ and ‘.test’ are controlled by smart contracts known as registrars, which define the logic for assigning their subdomains. By complying with the requirements set in those registrar contracts, anyone can secure ownership of a domain for their own use. ENS further allows importing DNS names that the user already owns so they can be utilized within ENS.

Given ENS’s hierarchical structure, any domain holder can set up subdomains at any level, either for themselves or for others, as they see fit. For example, if Alice controls 'alice.eth', she can create 'pay.alice.eth' and set it up according to her preferences.

ENS is live on the Ethereum main network as well as multiple test networks. If you work with a library like the ensjs Javascript library-or an end-user application-it will detect which network you’re using and then reference the ENS deployment for that specific network.