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DNS is the acronym for Domain Name System. The reverse DNS lookup is according an IP address to look up the corresponding domain name, which is just the opposite of the forward DNS lookup we usually perform to access Internet sites.

Because the reverse DNS lookup is to query the domain name based on the IP address, and add the PTR (Pointer Record) record of your IP address to the relevant IP authorized DNS server, whether the identity of this IP address is recognized and legal can be judged through the reverse DNS lookup. The Internet is so extensive and virtual that gives opportunities to the evildoers to generate amount of Internet rubbish. Therefore, it is very necessary to perform a reverse DNS lookup.

Reverse DNS lookup is often used to filter spam. Spammers generally send emails with an invalid IP address that does not match the domain name. When performing reverse DNS lookup, you need to look up this IP address in a DNS database. If no valid domain name matching the IP address is found, the server will consider the email spam.

In Windows system, you can use “nslookup” instruction to make reverse DNS lookup. The steps are as follows:

① Press and hold the Win+R key at the same time, and enter “cmd”. Run this command to open the command prompt program.

② Enter "nslookup +IP" in the command window, for example, enter "nslookup 66.249.64.145", the query result is: crawl-66-249-64-145.googlebot.com, which is the IP address of the google spider. The query result is different from the IP address of 66.249.64.145, it means IP 66.249.64.145 has setup reverse DNS lookup.