Classfull Vs classless address

Classfull addressing:

            All IP addresses have a network and host portion. In classful addressing, the network portion ends on one of the separating dots in the address (on an octet boundary). Classful addressing divides an IP address into the Network and Host portions along octet boundaries.In the classful addressing system all the IP addresses that are available are divided into the five classes A,B,C,D and E, in which class A,B and C address are frequently used because class D is for Multicast and is rarely used and class E is reserved and is not currently used.

Each of the IP address belongs to a particular class that's why they are classful addresses.Earlier this addressing system did not have any name,but when classless addressing system came into existence then it is named as Classful addressing system.The main disadvantage of classful addressing is that it limited the flexibility and number of addresses that can be assigned to any device.One of the major disadvantage of classful addressing is that it does not send subnet information but it will send the complete network address. The router will supply its own subnet mask based on its locally configured subnets. As long as you have the same subnet mask and the network is contiguous, you can use subnets of a classful network address.

Classless Addressing:
Classless addressing uses a variable number of bits for the network and host portions of the address.

Classless addressing treats the IP address as a 32 bit stream of ones and zeroes, where the boundary between network and host portions can fall anywhere between bit 0 and bit 31.Classless addressing system is also known as CIDR(Classless Inter-Domain Routing).Classless addressing is a way to allocate and specify the Internet addresses used in inter-domain routing more flexibly than with the original system of Internet Protocol (IP) address classes.CIDR (Classless Internet Domain Routing), defines arbitrarily-sized subnets solely by base address and number of significant bits in the address. A CIDR address of 192.168.0.0/24 defines a block of addresses in the range 192.168.0.0 through 192.168.0.255, while 192.168.0.0/20 would define a network 16 times as large - from 192.168.0.0 through 192.168.15.255.

Decimal	192	160	20	48
Binary	11000000	10100000	00010100	0011	0000
	<-------- 28 bits Network ------->				4 bits host

What is Public IP And Private IP

PUBLIC IP:

      Public IP Addresses (also known as Static IP Addresses) are IP addresses that are visible to the public.Because these ip addresses are public, they allow other people to know about and access your computer, like a Web server.Public Ip is the IP which is visible in WAN means which goes out as a representative of our computer In some cases, you do not want people to access your computer or you want to restrict certain individuals from accessing your computer or server. A public IP address makes your equipment accessible to everyone on the internet and is needed for VoIP or if you want to give others to access to specific equipment on your network.  You must therefore be careful to protect your equipment from hackers and viruses and not to allow your equipment to be hi-jacked and used as an open email-relay, for example. 

Private IP:

       These addresses can be used on a private network, but they’re not routable through the public Internet. This not only creates a measure of much-needed security, but it also conveniently saves valuable IP address space.
private ips are the ips which we use within a company or organisation. A private IP address is for private use within the network and allows many more PCs to be connected.  If a customer is using a private IP and later wants VOIP they would need to change to a public IP address.

IP Addressing for Networking

Every computer on the Internet has a  unique IP address.Withe the help of that uniquely IP we identifies the device and found it from other computers on the Internet.IP address consists of 32 bit , often shown as 4 octets of numbers from 0-255 represented in decimal form instead of binary form.
 For Example the IP Address is                                192.168.1.125
                                                               11000000.10101000.00000001.01111101
       For human being it is easier to remember the decimal number rather then the binary code.Fot that reson we use the decimal to represent the ip address.but the binary represtation is important to detemine in which class this ip belong with.IP address always assigne according to class.i mean for ip addresses there is a standard that apply according to your requirement.IP classes define how many network ids and how many host this network actually has.

Now we discuse ip classes

Historically, RFC1700 grouped the unicast ranges into specific sizes called class A, class B, and class C addresses. It also defined class D (multicast) and class E (experimental) addresses, as previously presented. The unicast address classes A, B, and C defined specifically-sized networks as well as specific address blocks for these networks, as shown in this figure.

how we get the number of network and number of host per network.see the following table

Note: Class A addresses 127.0.0.0 to 127.255.255.255 cannot be used and is reserved for loopback and diagnostic functions.