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.
Terminology:
· Bit - A bit is one digit, either a 1 or a 0.
· Byte - A byte is 8 bits.
· Octet - An octet, made up of 8 bits, is just an ordinary 8-bit binary number.
· 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 192.168.1.0.
· 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.
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 172.20.18.125
· Binary, as in 10101100.00010100.00010010.01111101
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 network address (which can also be called the network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. In the IP address 172.20.18.125, for example, 172.20 is the network address. The node address is assigned to, and uniquely identifies, each machine on a network. This part of the address must be unique because it identifies a particular machine - an individual - as opposed to a network, which is a group. This number can also be referred to as a host address. In the sample IP address 172.20.18.125, the 18.125 is the node address. The designers of the Internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the rank Class A network. At the other extreme is the Class C network, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the Class B network.
Class A | Class B | Class C | |
First Octet Range | 1 to 126 | 128 to 191 | 192 to 223 |
Valid Network Numbers | 1.0.0.0 to 126.0.0.0 | 128.1.0.0 to 191.254.0.0 | 192.0.1.0 to 223.255.254.0 |
Number of Networks in This Class | 27 - 2 | 214 - 2 | 221 - 2 |
Number of Hosts per Network | 224 - 2 | 216 - 2 | 28 - 2 |
Size of Network Part of Address (bytes/bits) | 1 / 8 | 2 / 16 | 3 / 24 |
Size of Host Part of Address (bytes/bits) | 3 / 24 | 2 / 16 | 1 / 8 |
Default Mask | 255.0.0.0 | 255.255.0.0 | 255.255.255.0 |
By definition, an IP address that begins with 8 in the first octet is in a Class A network, so the network part of the address is the first byte, or first octet. An address that begins with 130 is in a Class B network. By definition, Class B addresses have a 2-byte network part, as shown. Finally, any address that begins with 199 is in a Class C network, which has a 3-byte network part. Also by definition, a Class A address has a 3-byte host part, Class B has a 2- byte host part, and Class C has a 1-byte host part.
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