IP addressing

IP Addressing
One of the most important topics in any discussion of TCP/IP is IP addressing. An IP address is a numeric identifier assigned to each machine on an IP network. It designates the specific location of a device on the network.
An IP address is a software address, not a hardware address—the latter is hard-coded on a network interface card (NIC) and used for finding hosts on a local network. IP addressing was designed to allow hosts on one network to communicate with a host on a different network regardless of the type of LANs the hosts are participating in.
Before we get into the more complicated aspects of IP addressing, you need to understand some of the basics. First I’m going to explain some of the fundamentals of IP addressing and its terminology.
Then you’ll learn about the hierarchical IP addressing scheme and private IP addresses.
IP Terminology
Throughout this chapter you’ll learn several important terms vital to your understanding of
the Internet Protocol. Here are a few to get you started:
Bit A bit is one digit, either a 1 or a 0.
Byte A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits.
Octet An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable.
Network address This is the designation used in routing to send packets to a remote network— for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0.
Broadcast address The address used by applications and hosts to send information to all nodes on a network is called the broadcast address. Examples include 255.255.255.255, which is all networks, all nodes; 172.16.255.255, which is all subnets and hosts on network 172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.
The Hierarchical IP Addressing Scheme
An IP address consists of 32 bits of information. These bits are divided into four sections, referred to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP address using one of three methods:
_ Dotted-decimal, as in 172.16.30.56
_ Binary, as in 10101100.00010000.00011110.00111000
_ Hexadecimal, as in AC.10.1E.38
All these examples truly represent the same IP address. Hexadecimal isn’t used as often as
dotted-decimal or binary when IP addressing is discussed, but you still might find an IP address stored in hexadecimal in some programs. The Windows Registry is a good example of a program that stores a machine’s IP address in hex.
The 32-bit IP address is a structured or hierarchical address, as opposed to a flat or nonhierarchical address. Although either type of addressing scheme could have been used, hierarchical addressing was chosen for a good reason. The advantage of this scheme is that it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with two possible values for each position—either 0 or 1—gives you 232, or 4,294,967,296). The disadvantage of the flat addressing scheme, and the reason it’s not used for IP addressing, relates to routing. If every address were unique, all routers on the Internet would need to store the address of each and every machine on the Internet. This would make efficient routing impossible, even if only a fraction of the possible addresses were used.