Default Reverse Lookup name for unused IP in unbound

[JAHANZAIB SYED]

If I query reverse look via any public dns for my ISP's  different Ip address, I get  this result,,,

For every IP on which my ISP have made custom record I get this result:
** nslookup 111.111.111.158
nslookup 111.111.111.159.myisp.com
Name:    company1.isp.com
** nslookup 111.111.111.159
nslookup 111.111.111.159.myisp.com
Name:    company2.isp.com

For several other hundreds IP addresses of my ISP, I get this result as default

** nslookup 111.111.111.160
Name:    111-111-111-160.myisp.com
Address:  111.111.111.160


** nslookup 111.111.111.161
Name:    111-111-111-161.myisp.com
Address:  111.111.111.161

How this is done that every unused ip, no ping replies from them and each ip is showing isp name by default? Do ISP creates manual records for there unused IP's as well or is there any geenral statemetn defined in there dns?
How I can do this in my Unbound config so that if I do Nslookup for any unused Ip of my LAN it should return like x-x-x-x-mydomain.com ?


Regards,
SJZ
<http:///>
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