delegated-registrar by 1st class-A network:-
- http://www.iana.org/assignments/ipv4-address-space
specific ip address, visit the respective IP registrar
APNIC (Asia Pacific):-
- http://www.apnic.net, use their whois tool
ARIN (Americas and Africa)
- http://www.arin.net, use their whois tool
RIPE (Europe and surrounding)
- http://www.ripe.net