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What is an IP Address?

Internet Protocol Address (or IP Address) is an unique address that computing devices such as personal computers,
tablets, and smartphones use to identify itself and communicate with other devices in the IP network. Any device
connected to the IP network must have an unique IP address within the network. An IP address is analogous to a street
address or telephone number in that it is used to uniquely identify an entity.

Dotted Decimals

The traditional IP Addresses (known as IPv4) uses a 32-bit number to represent an IP address, and it defines both
network and host address. A 32-bit number is capable of providing roughly 4 billion unique numbers, and hence IPv4
addresses running out as more devices are connected to the IP network. A new version of the IP protocol (IPv6) has been
invented to offer virtually limitless number of unique addresses. An IP address is written in "dotted decimal"
notation, which is 4 sets of numbers separated by period each set representing 8-bit number ranging from (0-255).

An IPv4 address is divided into two parts: network and host address. The network address determines how many of the 32
bits are used for the network address, and remaining bits for the host address. The host address can further
divided into subnetwork and host number.

Class A, B, C and CIDR networks

Traditionally IP network is classified as A, B or C network. The computers identified the class by the first 3 bits
(A=000, B=100, C=110), while humans identify the class by first octet(8-bit) number. With scarcity of IP addresses, the
class-based system has been replaced by Classless Inter-Domain Routing (CIDR) to more efficiently allocate IP

Public and Private IP Addresses

In order to maintain uniqueness within global namespace, the IP addresses are publicly registered with the Network
Information Center (NIC) to avoid address conflicts. Devices that need to be publicly identified such as web or mail
servers must have a globally unique IP address, and they are assigned a public IP address. Devices that do not
require public access may be assigned a private IP address, and make it uniquely identifiable within one organization.
For example, a network printer may be assigned a private IP address to prevent the world from printing from it. To
allow organizations to freely assign private IP addresses, the NIC has reserved certain address blocks for private
use. A private network is a network that uses RFC 1918 IP address space.

What is loopback IP address?

The loopback IP address is the address used to access itself. The IPv4 designated as the loopback address with
the subnet mask. A loopback interface is also known as a virtual IP, which does not associate with
hardware interface. On Linux systems, the loopback interface is commonly called lo or lo0. The corresponding hostname
for this interface is called localhost.

The loopback address is used to test network software without physically installing a Network Interface Card (NIC), and
without having to physically connect the machine to a TCP/IP network. A good example of this is to access the web
server running on itself by using or http://localhost.