Probably, you have heard these words many times, but have no idea what IP address is.
Internet Protocol address is a unique address that is given to any computing device (PC, smartphone or tablet) that is used to identify it and establish the communication with other devices in the Network. A device is assigned own IP address every time it is connected to the Internet. In its essence, this is the same thing as a phone number or street address, serving to identify the user and his devices.
As a rule, a traditional IP address (called IPv4) is a 32-bit number that defines both host address and network. But it can provide 4 billion unique numbers only, which is not sufficient, taking into consideration the growing amount of devices connected to the Internet. The new standard (IPv6) has been invented recently to offer practically unlimited number of unique addresses. It is written in "dotted decimal" notation, presenting four sets of numbers separated by period (each set is comprised of a 8-bit number ranging from 0 to 255).
For the addresses to remain unique and avoid conflicts, they are publicly registered in Network Information Center (NIC). Publicly identified devices (web or mail servers with globally unique IP address) are given a unique IP. And the devices that are identifiable within one organization are assigned a private IP unique on the frames of this certain organization to restrict access to them.