IP Addressing

A name indicates what we seek. 

                An address indicates where it is. 

                                   A route indicates how to get there...

• IP Addresses are globally unique, 32-bit numbers, normally represented as four sets of octets. In the dotted-decimal notation, an ip address could look like: 171.32.12.2

• As far as the devices on the network are concerned, an ip address is just a 32-bit binary number.

• IP addresses are broken down into two parts, a network identifier and a host identifier.

CLASSFUL ADDRESSING:

Class	Class Identifier	Network Prefix	Host Portion
A	First octet in the range 1–126*	First octet N.H.H.H or 1.xxx.xxx.xxx to 126.xxx.xxx.xxx	Remaining three octets N.H.H.H or xxx.0.0.0 to xxx.255.255.255
B	First octet in the range 128–191	First two octets N.N.H.H or 128.0.xxx.xxx to 191.255.xxx.xxx	Remaining two octets N.N.H.H or xxx.xxx.0.0 to xxx.xxx.255.255
C	First octet in the range 192–223**	First three octets N.N.N.H or 192.0.0.xxx to 223.255.255.xxx	Remaining octet N.N.N.H or xxx.xxx.xxx.0 to xxx.xxx.xxx.255

• Class D addresses are for multicasting and Class E addresses are reserved for future use.

Addresses starting with 0 and 127 are reserved:

- The all-zeros address is reserved as the default network. This is used by routers as a way to identify where to send a packet when there is no match for it in a routing table.

- Addresses starting with 127 are loopback addresses. That means it points to your own device.   

So why do we need loopback addresses? Why send packets to your own self? Do we really need this?

          YES, we do! Here's why:

*  It can be used by a network client software on a device to talk to server (web server, maybe). This helps in testing services without external network exposure.

*  Pinging the loopback interface is the first test to check if the functionality of the IP stack is working.

IMP: A unique use of the loopback network addresses is their part in MultiProtocol Label Switching (MPLS) traceroute error detection techniques. Here, their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users.

• Any IP address with all the Host Bits set to zero - indicates the network itself.
• Any IP address with all the Host Bits set to one - indicates the broadcast address for that network.

• To determine the number of possible networks or hosts, following is the formula:

                             2^n – 2, where n is the number of bits in the network or host space. 
          

          The subtraction of 2 is for the network address and broadcast address.

Some networks are reserved and cannot be used on the internet:

Class	Address Range
A	10.x.x.x
B	172.16.x.x through 172.31.x.x
C	192.168.x.x

These address are used for (1) testing and for (2) corporate intranets.


This CLASSFUL ADDRESSING, however, does not allow for efficient use of the available addresses. Networks are either too big or too small - hence, unmanageable. CLASSLESS ADDRESSING, on the other hand, allows networks to be divided into smaller, manageable networks. This is done using the concept of SUBNETTING.




CLASSLESS ADDRESSING:


Classless Addressing allows for a hierarchical architecture of IP addresses. The Internet, as a whole is at the top, whereas, the levels below are the smaller networks / sub-networks in it.



To use sub-netting, the host portion is broken up into two parts:
 - sub-network part
 - host part


But since the network part is the only part which is significant to the internet, the subnets are not visible outside of the private network of local organizations.
Hence, the route from the internet to any subnet of a given IP address is the same, irrespective of which subnet the destination host lies in. The local routers will then decide which subnet to route the packets to.


This has two advantages (and they are the very reasons for the classless routing concept):


-- It reduces the complexity of the routing table, since only one network address is required to reach an organization.
-- This also prevents the depletion of available network addresses because each data link does not need to take up a full IP network.



SUBNET MASK:


- is a mask used to determine which subnet an IP address belong to. 


It is the network address plus the bits reserved for identifying the subnet. 
It can be used to identify the subnet to which a particular IP address belongs by doing a bitwise AND operation on the mask and the IP address. The result of this operation is the subnetwork address:


Subnet Mask        255.255.248.0            11111111.11111111.11111000.00000000
IP Address           145.115.9.11              10010001.01110011.00001001.00001011
Subnet Address   145.115.8.0                 10010001.01110011.00001000.00000000
The subnet address is 145.115.8.0