It's like a telephone number. It contains four groups of numbers from 0 to 255 separated by a period. Every time a computer accesses a site or sends an email, it's IP address is reported to the server. For example, your IP address was reported to this server as "18.216.208.243".

Like telephone numbers there are directories that relate the IP addresses to names such as "cnn.com". These names are called domain names and the directories are run on servers called Domain Name Servers or DNS. A Reverse DNS Lookup queries these servers in an attempt to find a match.

Just like there are unlisted telephone numbers there are unlisted IP addresses. For example, an Internet Service Provider (ISP) may be assigned a block of IP addresses and it may not give names to all those addresses.

It depends. You can be resonable sure in assuming that the Domain Name returned by a Reverse DNS Lookup is correct but the real question is "Is the IP Address I looked up the real IP address?"

An innocent user visiting your web site or sending you an email doesn't usually have anything to hide. A spammer or hacker trying to break into your computer or firewall has a lot to hide. There are many techniques they can use to hide or change their apparent IP Address. For example, there are many anonymous proxies which allow people to visit web site and hide their real IP Address. In the case of spam, some spam is sent by viruses which infect computers and hijack them to send email. It is also possible for spammers and hackers to modify IP packets to change their reported IP Address.

Bottom line: you can't always trust that the IP Address you think you're looking up is the correct one.

Once you obtain the domain name you need to determine who it belongs to. The "Whois" database on the internet contains information about the person or organization who registered the domain name. You'll be given the option to query this database for many of the domains after resolving the IP address on this site.

While ICANN, the international organization that governs and polices the issuance of domain names, requires that this information be on file and up-to-date there is no guarantee that this is the case.

Depending on the nature of your issue with the owner of the domain name you could try contacting them at the address or telephone number given in the "whois" database. In many cases the following email addresses on the domain name are monitored and responsed to: webmaster, abuse, spam and spoof.

Good luck!