问题
bbs.example.com
I referred to the domain name knowledge on Wikipedia, and have read through the page, but I still don't know what is a second-level domain name? The page says com is a TLD (Top Level Domain), and the TLDs are the highest level of domain names on the Internet.
It says. bbs.example.com is a third-level domain name? But someone told me bbs.example.com is a second-level domain name. .com is the TLD, not the domain name.
Which is right? Thank you.
回答1:
bbs in bbs.example.com is a third-level domain name. example in example.com is a second-level domain name. com is a top-level domain name. (TLD)
回答2:
My answer is maybe too late, but I wanted to add some details to @icktoofay's correct answer.
The Domain Name System is a hierarchical naming system (tree). The hierarchy is represented by each part separated by a dot, that are interpreted from right to left for example: bbs.example.com. should be interpreted as .com.example.bbs and decomposed as:
- Layer 1:
.comis a Top Level Domain - Layer 2:
.com.exampleis a Second Level Domain - Layer 3:
.com.example.bbsis a Third Level Domain
That's where the notion of top, second and third level are coming from.
I wrote an article on my personal blog introducing the domain name system if you are interested where I describe this in more detail: What's a domain name and what's behind the scene
回答3:
Use the separate list of tlds from https://publicsuffix.org/list/public_suffix_list.dat
This is maintained by mozilla.
By using this list of tld you can find the domain level.
Let me know if you have any more queries.
来源：https://stackoverflow.com/questions/6555572/is-this-domain-a-second-level-domain-name