What is an IP address?

What is an IP address?
Every device connected to the public Internet is assigned a unique number known as an
Internet Protocol (IP) address. IP addresses consist of four numbers separated by periods (also
called a 'dotted-quad') and look something like 127.0.0.1.
In computer networking, an Internet Protocol (IP) address consists of a numerical
identification (logical address) that network management assigns to devices participating in a
computer network utilizing the Internet Protocol for communication between its nodes.
Although computers store IP addresses as binary numbers, they often display them in more
human-readable notations, such as 192.168.100.1 (for IPv4), and 2001:db8:0:1234:0:567:1:1
(for IPv6). The role of the IP address has been characterized as follows: "A name indicates
what we seek. An address indicates where it is. A route indicates how to get there." [2]
 

What is subnet Mask ?
A subnet (short for "subnetwork") is an identifiably separate part of an organization's
network. Typically, a subnet may represent all the machines at one geographic location, in
one building, or on the same local area network (LAN). Having an organization's network
divided into subnets allows it to be connected to the Internet with a single shared network
address. Without subnets, an organization could get multiple connections to the Internet, one
for each of its physically separate subnetworks, but this would require an unnecessary use of
the limited number of network numbers the Internet has to assign. It would also require that
Internet routing tables on gateways outside the organization would need to know about and
have to manage routing that could and should be handled within an organization.
 
What is ARP? What is ARP Cache Poisoning?
Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol address (IP
address) to a physical machine address that is recognized in the local network. For example,
in IP Version 4, the most common level of IP in use today, an address is 32 bits long. In an
Ethernet local area network, however, addresses for attached devices are 48 bits long. (The
physical machine address is also known as a Media Access Control or MAC address.) A
table, usually called the ARP cache, is used to maintain a correlation between each MAC
address and its corresponding IP address. ARP provides the protocol rules for making this
correlation and providing address conversion in both directions.
 
How ARP Works
When an incoming packet destined for a host machine on a particular local area network
arrives at a gateway, the gateway asks the ARP program to find a physical host or MAC
address that matches the IP address. The ARP program looks in the ARP cache and, if it finds
the address, provides it so that the packet can be converted to the right packet length and
format and sent to the machine. If no entry is found for the IP address, ARP broadcasts a
request packet in a special format to all the machines on the LAN to see if one machine
knows that it has that IP address associated with it. A machine that recognizes the IP address
as its own returns a reply so indicating. ARP updates the ARP cache for future reference and
then sends the packet to the MAC address that replied.