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Rules for IP Addresses

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 has at least one interface with an IP address can send and receive IP packets
and is called an IP host.

IP addresses consist of a 32-bit number, usually written in dotted-decimal notation (DDN).
The “decimal” part of the term comes from the fact that each byte (8 bits) of the 32-bit IP
address is shown as 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 example, 168.1.1.1 is an IP address written in dotted-decimal form;
the actual binary version is 10101000 00000001 00000001 00000001. (You almost never
need to write down the binary version, but you can use the conversion chart in Appendix A,
“Numeric Reference Tables,” to easily convert from DDN to binary or vice versa.)

Each DDN has four decimal octets, separated by periods. The term octet is just a vendorneutral
term for byte. Because each octet represents an 8-bit binary number, the range of
decimal numbers in each octet is between 0 and 255, inclusive. For example, the IP address
of 168.1.1.1 has a first octet of 168, the second octet of 1, and so on.

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. For example, if your laptop has both an Ethernet network interface card
(NIC) and a wireless NIC, with both working at the same time, both will have an IP address.
Similarly, routers, which typically have many network interfaces that forward IP packets,
have an IP address for each interface.

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