This document defines key concepts for domain name services, including domain names, top-level domains, second-level domains, third-level domains, domestic domain names, domain name registrars, and domain name registries.
Domain name
A domain name is a unique, human-readable name that identifies a computer or group of computers on the Internet. Composed of a dot-separated string of characters, it specifies a resource's online location.
To visit a website or web application, enter its domain name in your web browser's address bar. For example, you use www.aliyun.com to access Alibaba Cloud services.
To make your website or web application accessible on the Internet, you must first register a domain name.
Top-level, second-level, and third-level domains
The Internet Corporation for Assigned Names and Numbers (ICANN) manages and coordinates the global Domain Name System. According to ICANN, a complete domain name consists of at least two parts, or labels, separated by dots (.). The label to the right of the last dot is the top-level domain (TLD), also known as the first-level domain. The label to the left of the TLD is the second-level domain. The label to the left of the second-level domain is the third-level domain, and so on. Each level in the hierarchy controls the assignment of domains in the level below it. The following figure uses www.example.com.cn to illustrate these levels.
The following table details each level.
|
Domain level |
Description |
Example |
|
Top-level domain |
Top-level domains are categorized as follows.
|
|
|
Second-level domain |
The domain label immediately to the left of a top-level domain. A directory path such as |
In |
|
Third-level domain |
The domain label immediately to the left of a second-level domain. A third-level domain is a type of subdomain. |
In |
-
When registering a domain name, the customizable part precedes the domain suffix. Depending on the registry's rules, the available suffix might be a top-level domain (such as
.cn) or a compound suffix that includes a second-level domain (such as.com.cn,.net.cn, or.org.cn). -
In the context of DNS resolution, the terminology can differ from the structural definition. To simplify configuration, the complete domain name that you register is considered the first-level domain (not to be confused with a top-level domain). The first label to the left of this registered domain (a subdomain) is considered the second-level domain, and so on.
You cannot register a subdomain, such as blog.example.com, separately. You must first register the root domain, such as example.com, and then use Alibaba Cloud DNS to add a DNS record for the subdomain. For detailed instructions, see Add a DNS record.
Www prefix and subdomains
The www prefix, an abbreviation for World Wide Web, is a common subdomain that typically identifies a website's main page.
The www prefix is optional. You can access a website directly using the root domain, such as aliyun.com. Using www is a convention to denote the primary web service.
You can create multiple websites by setting up different subdomains under the same root domain. A subdomain is placed to the left of the root domain and separated by a dot (.). For the root domain yuntech.site, you can create the following subdomains:
-
www.yuntech.site: As the default address for the main website. -
blog.yuntech.site: For a blog site. -
shop.yuntech.site: For an e-commerce page. -
api.yuntech.site: For API service endpoints.
After creating a subdomain, you must configure a DNS record to point it to a server address. A subdomain's DNS resolution settings are independent of its root domain, allowing it to point to a different server or service.
Domestic domain names
In Alibaba Cloud Domain Name Service, "domestic domain name" refers to domain names with one of the following four suffixes: .cn, .xn--fiqs8s (中国), .xn--55qx5d (公司), and .xn--io0a7i (网络). All references to "domestic domain name" in the help documentation for domain names adhere to this definition.
Domain name registrar
A domain name registrar handles domain name registrations for specific top-level domains (TLDs). It is a commercial entity or organization accredited by the Internet Corporation for Assigned Names and Numbers (ICANN) or designated by a country code top-level domain (ccTLD) registry.
Domain name registry
A domain name registry is the organization that manages all domain names under a specific top-level domain. For example, VeriSign is the registry for the .com TLD. A domain name registry can define the rules for domain name registration, such as residency requirements for registering a domain under a specific ccTLD.
A domain name registry also maintains the authoritative database for all domains under a TLD. This database contains information such as each domain's contact details and designated name servers.