IP ADDRESSES

An identifier for a computer or device on a TCP/IP Network. Networks using the TCP/IP protocol route messages based on the IP address of the destination. The format of an IP address is a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 1.160.10.240 could be an IP address.

Within an isolated network, you can assign IP addresses at random as long as each one is unique. However, connecting a private network to the Internet requires using registered IP addresses (called Internet addresses) to avoid duplicates.

The four numbers in an IP address are used in different ways to identify a particular network and a host on that network. Four regional Internet registries -- ARIN, RIPE NCC, LACNIC and APNIC -- assign Internet addresses from the following three classes.

· Class A - supports 16 million hosts on each of 126 networks

· Class B - supports 65,000 hosts on each of 16,000 networks

· Class C - supports 254 hosts on each of 2 million networks

The number of unassigned Internet addresses is running out, so a new classless scheme called CIDR is gradually replacing the system based on classes A, B, and C and is tied to adoption of. An IP Address is the numerical address of a computer on the Internet. This means every personal computer on the Internet will be given a unique IP Address by their Internet Service Provider just as every web site is given an IP Address by their web site host

Hosts and networks

IP addressing is based on the concept of hosts and networks. A host is essentially anything on the network that is capable of receiving and transmitting IP packets on the network, such as a workstation or a router. It is not to be confused with a server: servers and client workstations are all IP hosts.

The hosts are connected together by one or more networks. The IP address of any host consists of its network address plus its own host address on the network.

IP addressing, unlike, say, IPX addressing, uses one address containing both network and host address. How much of the address is for the network portion and how much for the host portion varies from network to network.

An IP address is 32 bits wide, and as discussed, it is composed of two parts: the network number, and the host number [1, 2, 3].

By convention, it is expressed as four decimal numbers separated by periods, such as "200.1.2.3" representing the decimal value of each of the four bytes. Valid addresses thus range from 0.0.0.0 to 255.255.255.255, a total of about 4.3 billion addresses. The first few bits of the address indicate the Class that the address belongs to:

Class
Prefix
Network Number
Host Number

A
0
Bits 0-7
Bits 8-31

B
10
Bits 1-15
Bits 16-31

C
110
Bits 2-24
Bits 25-31

D
1110
N/A


E
1111
N/A
 

The bits are labeled in network order, so that the first bit is bit 0 and the last is bit 31, reading from left to right. Class D addresses are multicast, and Class E are reserved. The range of network numbers and host numbers may then be derived:

Class
Range of Net Numbers
Range of Host Numbers

A
0 to 126
0.0.1 to 255.255.254

B
128.0 to 191.255
0.1 to 255.254

C
192.0.0 to 254.255.255
1 to 254

Any address starting with 127 is a loop back address and should never be used for addressing outside the host. A host number of all binary 1's indicates a directed broadcast over the specific network. For example, 200.1.2.255 would indicate a broadcast over the 200.1.2 network. If the host number is 0, it indicates "this host".

If the network number is 0, it indicates "this network" [2]. All the reserved bits and reserved addresses severely reduce the available IP addresses from the 4.3 billion theoretical maximum. Most users connected to the Internet will be assigned addresses within Class C, as space is becoming very limited. This is the primary reason for the development of IPv6, which will have 128 bits of address space.

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