How can I successfully lookup hostname from an IP, but not vice versa?

Question

Have a look at the following commands and their respective outputs:

$ nslookup 202.83.21.28 8.8.8.8
Server:  google-public-dns-a.google.com
Address:  8.8.8.8

Name:    broadband.actcorp.in
Address:  202.83.21.28


$ nslookup broadband.actcorp.in 8.8.8.8
Server:  google-public-dns-a.google.com
Address:  8.8.8.8

*** google-public-dns-a.google.com can't find broadband.actcorp.in: Non-existent domain

Why can I resolve the IP address 202.83.21.28 to its hostname broadband.actcorp.in but not vice versa? The way I understand nameservers work is, there should simply be an entry like broadband.actcorp.in <-> 202.83.21.28, or something like that. What am I missing?

Answer 1

... there should simply be an entry like broadband.actcorp.in <-> 202.83.21.28, or something like that.

No, in general there are (at least) two entries:

• An A record for broadband.actcorp.in to 202.83.21.28. This is how forward name resolution is performed.

• A PTR record for 28.21.83.202.in-addr.arpa to broadband.actcorp.in. This is how reverse resolution is performed.

If either entry is missing then the corresponding resolution will fail.