Internet, Networking, & Security Home Networking IP Address Forward and Reverse DNS Lookup URLs and IP addresses are two sides of the same coin By Bradley Mitchell Writer An MIT graduate who brings years of technical experience to articles on SEO, computers, and wireless networking. our editorial process LinkedIn Bradley Mitchell Updated February 27, 2020 Juan Silva / Getty Images Home Networking The Wireless Connection Routers & Firewalls Network Hubs ISP Broadband Ethernet Installing & Upgrading Wi-Fi & Wireless Tweet Share Email In networking, IP address lookup is 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 address number to the name. For most 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 a computing device such as a computer, smartphone, or tablet to identify it on a network. IPv4 addresses are 32-bit numbers and provide about 4 billion possible numbers. The newest version of the IP protocol (IPv6) offers an almost limitless number of unique addresses. For example, an IPv4 address looks like 220.127.116.11; 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 to remember and is susceptible to typographical errors. This is why URLs are used to go to websites. URLs are easier to remember and type correctly. Behind the scenes, however, a URL is translated to a corresponding numerical IP address to load the requested website. Typically, the URL (commonly called a website address) is entered into a web browser on a 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 process is invisible to the user, who sees only the website corresponding to the URL 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 Before dynamic IP addressing, many small business networks lacked name servers. These networks managed private IP lookups through host files that contained lists of static IP addresses and associated computer names. This IP lookup mechanism is still used on some Unix computer networks. It is also used on home networks that lack routers and with static IP addressing. Dynamic Host Configuration Protocol (DHCP) automatically manages IP addresses within a network. DHCP-based networks rely on the DHCP server to maintain host 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 URL is entered. Using a range of IP addresses allows more people to view the website simultaneously. Utility programs provided with a computer network operating system allow IP address lookups on both private LANs and the internet. In Windows, for example, the nslookup command (entered in a Command Prompt window) supports lookups using name servers and host files. The command is the same for macOS and is entered in a Terminal window. Public nslookup sites on the internet include Kloth.net, Network-Tools.com, and CentralOps.net.