What is an IP Number?
'IP' stands for Internet Protocol - it decribes the way information is sent and received over the Internet. An IP number is a 32 bit number assigned to any device that wishes to communicate over the Internet. Typically the IP number is represented as a set of four numbers in the range 0-255 - an address is usually written as these four numbers separated by dots. It can also be shown as one big number in decimal or hexadecimal.
What is my IP Number?
The IP Number of this machine is:
This number can also be represented as a 32–bit decimal number 315452824, or as a 32–bit hexadecimal number 0x12CD6D98.
(Note that if you are part of an internal network then this is the IP number of your local server, the machine which is connected to the external internet.)
More info on How are IP Addresses used? and Private IP Addresses .
Network Address Translation
Typically a home or office network will have a router to connect it to the internet, with one external IP address. When a local device (having an IP in one of the ranges described in the previous section) issues a request to contact an IP on the internet, the router keeps a track of which device made the request, so that when the reply is received it can be forwarded to the correct destination on the local network.
The NAT process involves modifying outbound messages so that their source IP is the externally visible IP of the router, and the message must also include information (often encoded in the port number) on the internal IP and port of the machine issuing the request. When a reply is received, the information is decoded in order to forward the data to the correct local machine.
Also see How are IP Numbers Assigned? and What is IPv6?
What is DNS?
Most Internet users prefer to use addresses which are more meaningful than a sequence of numbers - for example 'www.microsoft.com' instead of '220.127.116.11'. The translation from the textual domain name to the numeric IP address is performed by certain machines on the internet, known as 'Domain Name Servers'. Every computer that wants to be able to use domain names needs to know the IP address of at least one Domain Name Server - it contacts this server any time it needs to translate a domain name to an IP address. The server can either reply immediately with the IP address if it knows it, or it can forward the request to another server which may be in a better position to provide the information. When the translation is successfully performed, the server will keep a copy of, or cache, the address so that it can answer the request more rapidly the next time it is asked.
Domain names are resolved starting with the final part, the Top Level Domain (e.g. '.com', or '.de'). Every Domain Name Server knows the locations of certain machines known as Root Servers, which are able to indicate where to find a machine that can handle the particular Top Level Domain. The remainder of the domain name is then sent to this machine, which knows where to locate servers that can deal with the next level in the domain name, and so on until the first part of the name is reached (e.g. 'www'). At this point a full IP address can be returned through the various servers, back to the machine which originated the translation request.
Each machine along the return journey will keep a cached copy of the IP address and domain name, so that if asked for the same address in the near future it can respond immediately without having to contact any other machines. This speeds up general operation of the network, although it does mean that if the IP address for a particular domain name is changed at any time, it can take a while - sometimes several days - for that change to be propagated through the network. Until the change is propagated, the name servers will continue to report the old (cached) IP address.
To find out where an IP address is located, use free tool: IP address lookup