IP Address Forward and Reverse DNS Lookup

URLs and IP addresses are two sides of the same coin

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In networking, IP address lookup refers to the process of translating between IP addresses and internet domain names. Forward IP address lookup converts an internet name to an IP address. Reverse IP address lookup converts the IP number to the name. For the vast majority of computer users, this process occurs behind the scenes.

What Is an IP Address?

An Internet Protocol Address (IP Address) is a unique number assigned to computing devices such as computers, smartphones and tablets. An IP address is used to identify a unique device and address. IPv4 addresses are 32-bit numbers, which can provide about 4 billion numbers. The newest version of the IP protocol (IPv6) offers an almost limitless number of unique addresses. 

For example, an IPv4 address looks like, while an IPv6 address looks like 2001:4860:4860::8844.

Why IP Address Lookup Exists

An IP address is a lengthy string of numbers that is difficult for any computer user to remember, and it is susceptible to typographical errors. Instead, computer users enter URLs to go to websites. The URLs are easier to remember and less likely to contain typographical errors. However, URLs must be translated to the corresponding lengthy numerical IP addresses, so the computer knows where to go.

Typical users type a URL into a web browser on their computer or mobile device. The URL goes to the router or modem, which performs a forward Domain Name Server (DNS) lookup using a routing table. The resulting IP address identifies the website the user wants to view. The process is invisible to the users who see only the website corresponding to the URL they type in the address bar.

Most users rarely need to be concerned with reverse IP lookups. They are used mostly for network troubleshooting, often to find out the domain name of an IP address that is causing a problem.

Lookup Services

Several internet services support both forward and reverse IP lookup for public addresses. On the internet, these services rely on the Domain Name System and are known as DNS lookup and reverse DNS lookup services.

In a school or corporate local area network, private IP address lookups are also possible. These networks use internal name servers that perform functions comparable to those of DNS servers on the internet. In addition to DNS, the Windows Internet Naming Service is another technology that can be used to build IP lookup services on private networks.

Other Naming Methods

Years ago, before the advent of dynamic IP addressing, many small-business networks lacked name servers and 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 still used on some Unix computer networks. It can also be used on home networks without a router and with static IP addressing in place.

Dynamic Host Configuration Protocol (DHCP) automatically manages IP addresses within a network. DHCP-based networks rely on the DHCP server to maintain hosts files. In many homes and small businesses, the router is the DHCP server. A DHCP server recognizes a range of IP addresses, not a single IP address. As a result, the IP address may differ the next time a user enters a URL. Using a range of IP addresses allows more people to view the website simultaneously.

Utility programs provided with a computer's network operating system allow IP address lookups on both private LANs and the internet. In Windows, for example, the nslookup command supports lookups via name servers and hosts files. There are also public nslookup sites on the internet including Name.space, Kloth.net, Network-Tools.com, and CentralOps.net.