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Internet Protocol Version 4 – IPv4 address Explanation and Understanding IP Addressing


Internet Protocol Version 4 – IPv4 address

Let talk something about IP protocol and IP addressing.

To be able to communicate on a private TCP/IP network or on the public Internet, each host on the network must be identified by a 32-bit IP address. These IP addresses can be grouped into two categories: public IP addresses and private IP addresses. Public addresses are the globally unique addresses that are connected to the Internet. Private addresses are confined to specific ranges that can be used by any private network. but that cannot be seen on the public Internet.

Public IP Addresses

Every IP address on the public Internet is unique. To allow networks to obtain unique addresses for the Internet, the Internet Assigned Numbers Authority (IANA) divides up the nonreserved portion of the IP address space and delegates responsibility for address allocation to a number of Regional Internet Registries throughout the world. Typically, your ISP assigns you one public IP address for each of your computers that is directly connected to the ISP. This IP address can be assigned dynamically to each computer when the computer connects, or it can be reserved statically for your dedicated line ( VPN , Leased line . Dial-up etc).

Private IP Addresses

The IANA has reserved a certain number of IP addresses that are never used on the global Internet. These private IP addresses are used for hosts that require IP connectivity but that do not need to be seen on the public network. For example, a user connecting computers in a home TCP/IP network does not need to assign a public IP address toeach host. The user instead can take advantage of the address ranges shown in following  Table to provide addresses for hosts on the network.

Starting Address        Ending Address           Class                                 No. of addresses                  A 24-bit block (/8 prefix,1xA)          16777216               B 20-bit block (/12 prefix,16xB)      1048576           C 16-bit block (/16 prefix,256xC)      65536

ipv4 address structure and classificationIP Addressing Methods -  IP addresses can be provided manually, dynamically by a DHCP server, or automatically by using Automatic Private IP Addressing (APIPA).

Manual ( Static ) IP Addressing

Occasionally, you need to assign addresses manually ( Static IP address ). For example, manual configuration is required in a network with multiple network segments and in which No DHCP server is present. In addition, if you want to set up a computer to be a DHCP server in a network, you should plan to assign its IP address manually. Finally, if you want   computer to serve as an essential network server, such as a DNS server, WINS server, or domain controller, you should plan to assign the servers a static IP address.

Dynamic Host Configuration Protocol ( DHCP )

A DHCP server automatically provides IP addresses to DHCP clients from the range orranges of available addresses you determine. You can also configure a DHCP server toassign clients automatically with other options, such as the addresses of DNS and WINSservers, gateway addresses, and other parameters.

Automatic Private IP Addressing ( APIPA )

APIPA or Link-Local address is an automatic addressing feature used for simple networks lacking a DHCP server and consisting of a single network segment . When DHCP server fails or is not present at all, APIPA allocates IP addresses in the private range to Clients verify their address is unique on the network using ARP. In APIPA, all devices use the default network mask and all reside on the same subnet

Alternate Configuration

Like APIPA, an Alternate configuration provides an IP address for computers unable to find a DHCP server. However, when you specify an alternate configuration on a given computer, that computer defaults (in the absence of a DHCP server) to the manually configured alternate address instead of an APIPA address. This feature is useful when the computer is used on more than one network and when one of the networks does not have a DHCP server. More details about Alternate IP address are here .

Structure of IP Addresses

People usually recognize IP addresses by their distinctive sequence of four numbers separated by dots, such as However, this version of an IP address is really just a transcription—called dotted-decimal notation—that people use to remember the address easily. Whereas decimal notation is the 10-digit numbering system that most people use every day, computer processing is based on binary notation, which uses only the digits 1 and 0 to represent all values. The native form of an IP address is thus binary.  The logic behind IP addressing is revealed when you look at this native binary version of IP addresses. To be able to configure, manage, and troubleshoot IP addressing, therefore, you must be able to understand and work with the binary form of IP addresses, as well as translate between binary and decimal notations.


Accustomed to decimal systems, people naturally prefer to express decimal values over their binary equivalents. With dotted decimal notation, each 32-bit address value of an IP address is viewed as four base-10 groupings ranging from 0 to 255, as in the example “”.  These values represent the four 8-bit values that make up the 32- bit address. In both binary and decimal notations, each of the four groupings is referred to as an octet. However, only in binary form is it possible to see the value of each individual bit. For example, the IP address is expressed in binary notation in the following manner :

11000000     10101000     00000000     11100001

For any 32-bit IP address, octets and bit places are numbered from left to right. Consequently, the first octet refers to the leftmost octet, and bit places 1 through 8 refer to the eight leftmost bit places, beginning on the far left. The second octet refers to the next eight bits (bit places 9–16), followed by the third octet (bit places 17–24), and the fourth octet (bit places 25–32). Periods are used to separate the four octets in dotted decimal notation, and spaces are used to separate them in binary notation.

Notice that as you move from left to right and begin with the first bit’s value of 128, each successive bit represents half the potential value of the previous bit. In reverse direction, as you move from right to left and begin with the eighth bit’s value of 1, each successive bit represents double the potential value of the previous bit. This pattern allows you to easily recall the potential value of each bit place within an octet. Note that these bit places represent values held only when a given bit place contains a 1. When an octet contains a 0 in any bit place, the value of the bit is 0. For example, if the first bit place is filled with a bit value of 1, the equivalent decimal value is 128. Where the bit value is 0, the equivalent decimal value is 0 as well. If all the bit places in an octet are filled with 1s, the equivalent decimal value is 255. If all the bit places are filled with 0s, the equivalent decimal value is 0.

In this eight-digit binary number the first, third, fifth, and sixth bit places contain 1. All other bit places are filled with 0. To understand the decimal value of this binary octet, you can easily draw a conversion table, such as the one shown here, in which to enter the potential bit values of the octet :

128      64      32       16        8       4       2       1

1           0         1          0        1       1       0       0

Using this table as a reference, you can perform simple addition of each bit place’s decimal equivalent value to find the decimal sum for this octet string, as follows:

first bit (128) + third bit (32) + fifth bit (8) + sixth bit (4) = octet total (172)

Because the sum is 172, the first octet of the example IP address is expressed as 172 in decimal form. Next, suppose that the following four octets represent the complete IP address:     10101100      00010001     00000111     00011011

After using this method on the other octets, you can determine that the complete dotted-decimal equivalent of the IP address is

In IPv4 version has 32 bit addressing structure and it provides more than 4 billion unique IP addresses. With the development of our fellow citizens from the Far East, China, Japan and India, for example, their need for Internet addresses now seem to have exhausted the possibilities.  It predicts that worldwide already exhausted all IPv4 unique addresses in some regions of the world. So with the announcement of Internet Protocol version 6IPv6  in 1995, changing gradually started addressing in order to extend the possibility comfortable all computer devices to join the net.