If a device wants to communicate using TCP/IP, it needs an IP address. When the device has an IP address and the appropriate software and hardware, it can send and receive IP packets. Any device that can send and receive IP packets is called an IP host.
IP addresses consist of a 32-bit number, usually written in dotted-decimal notation. The "decimal" part of the term comes from the fact that each byte (8 bits) of the 32-bit IP address is converted to its decimal equivalent. The four resulting decimal numbers are written in sequence, with "dots," or decimal points, separating the numbers—hence the name dotted-decimal. For instance, 188.8.131.52 is an IP address written in dotted-decimal form, but the actual binary version is 10101000 00000001 00000001 00000001. (You almost never need to write down the binary version—but you will need to know how to convert between the two formats in Chapter 12, "IP Addressing and Subnetting.")
Each of the decimal numbers in an IP address is called an octet. The term octet is just a vendor-neutral term instead of byte. So, for an IP address of 184.108.40.206, the first octet is 168, the second octet is 1, and so on. The range of decimal numbers numbers in each octet is between 0 and 255, inclusive.
Finally, note that each network interface uses a unique IP address. Most people tend to think that their computer has an IP address, but actually their computer's network card has an IP address. If you put two Ethernet cards in a PC to forward IP packets through both cards, they both would need unique IP addresses. Similarly, routers, which typically have many network interfaces that forward IP packets, have an IP address for each interface.
Now that you have some idea about the basic terminology, the next section relates IP addressing to the routing concepts of OSI Layer 3.
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