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Let’s show you something about IP Address. When IP and IP addresses were created in the 1960s, an IP address belonged to a particular class. This type of addressing was called classful addressing, and it was based, on five different classes. In approximately 1996, a new form of addressing became available: classless addressing. Although classless addressing is more common today, it is still important to understand how both of these systems work.
IP addresses are 32 bits long. In classful addressing, the addresses are not, however, simple 32-bit integers. Instead, they can consist of three specific pieces of information. The size and value of these three pieces of information depend on the basic form of the address. There are five forms of an IP address: Class A, B, C, D and E.
| Address Type | Beginning Bit Pattern | Network Address (net ID) | Host Address (host ID) | | Class A | 0 | 128 addresses (7 bits) | 16,777216 addresses(24 bits) | | Class B | 10 | 16,384 addresses (14 bits) | 65,536 addresses (16 bits) | | Class C | 110 | 2,097,152 addresses (21 bits) | 256 addresses (8 bits) | | Class D | 1110 | Multicast address | | | Class E | 1111 | Reserved addresses | |
Each IP address can consists of three parts:
- A 1-, 2-, 3-, or 4-bit identifier field (also known as a beginning bit pattern),
- A net ID, which indicates a particular network,
- A host ID, which indicates a particular host, or computer, on that network.
As you can see above, given a beginning bit pattern of 0, there are 12 Class A addresses, or networks, in existence. Each Class A address can have 16,777,216 hosts, or computer. Clearly, 128 is not very many networks; in fact, all the 128 Class A addresses were assigned a long time ago. Another impractical feature of the Class A address type is the allocation of 16,777,216 computers per network. Attaching 16,777,216 computers to one network are beyond imagination. As we probably know many local area networks rarely have more than a few hundred computers attached to them. Unfortunately, for this reason, many Class A addresses go unused. Class B addresses allow for 16,384 net IDs, or networks, each supporting 65,536 host IDs – meaning that each of the 16,384 networks can have 65,536 host computers attached to it. Class C addresses allow for 2,087,152 net IDs, or networks, and 256 host IDs. In the case of the Class C address type, the number of host computers allowed is to small to accommodate any networks but the smallest. Class D addresses are available for networks that allow multicasting of messages.
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