How efficient is DNS second-level domain lookup? For example, in a url web.example.com, the top-level domain is .com, and the second level domain is .example. When we query for web.example.com, the root servers will provide the gTLD servers for .com. After a gTLD server is selected for .com, this server will return the nameservers for example.com. How can one gTLD know the nameservers for all each possible second-level domain (such as “example.com”) since there could be so many possible second-level domains? Based on wikipedia (en.wikipedia.org/wiki/Domain_Name_System#Structure) each label is up to 63 characters, so if we limit to the english alphabet, this already gives us 26^63 possible second-level domains.
How efficient is DNS second-level domain lookup? For example, in a url web.example.com, the
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The reason is that very very few of the 26^63 domains are actually used.
Also, the DNS system is hierarchical, so once a DNS server at an ISP looked up for example cnn.com, it will cache the data and not ask the root server about it again for a set time even if other clients ask about it. After a while, many root domains are cached very near to the clients.
That is not to say that the root servers don’t have their work cut out for them… 🙂