Several Internet services support both forward and reverse IP lookup for public addresses. On the Internet, these services rely on the Domain Name System (DNS) and are known as DNS lookup and reverse DNS lookup services.→ See also - Top Free Internet DNS Servers
On a school or corporate local area network (LAN), private IP address lookups are also possible. These networks use internal name servers that perform the comparable functions to DNS servers on the Internet. In addition to DNS, the Windows Internet Naming Service (WINS) is another technology that can be used to build IP lookup services on private networks.
Years ago, before the advent of dynamic IP addressing, many smaller business networks lacking name servers managed private IP lookups through hosts files. Hosts files contained simple lists of static IP addresses and associated computer names. This IP lookup mechanism is sitll used on some Unix computer networks. It can also be used on home networks without a router and with static IP addressing in place. DHCP-based networks, like most home networks today, rely on the DHCP server to maintain hosts files.
Utility programs provided with the computer's network operating system (NOS) allow IP address lookups on both private LANs and the Internet. In Windows, for example, the nslookup command supports lookups via name servers and/or hosts files.