Classful and Classless IP Addressing

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Classful Addressing

The first major addressing scheme was published in September 1981 in RFC 790 by IETF. The IP addressing scheme was 32 bits long in size and addressing scheme were three classes A, B, and C, corresponding to 8-bit, 16-bit, and 24-bit prefixes. No other prefix lengths allowed at that time, and there was no concept of nesting a group of 24-bit prefixes, such as, within a 16-bit prefix.

The class D and class Class E addresses also defines, but, neither of these two address classes were normally used. Class D addresses are reserved for multicasting and Class E addresses are reserved for experimental and also for future use. The easiest way to distinguish between different address classes is to use the first decimal number in the IP address. Classful networks use the classful subnet mask according to the leading bits in the first block of the IP address. The figure below illustrates the key information of the Classful address scheme.


Class A ( to

The default subnet mask for this class is or /8. This class is support extremely large network with more than 16 million hosts. High order bit of Class A addresses are zero, so the remaining 7 bits in the first octet create total 128 possible class A network. is used for default router and network is reserved for local loop testing. So, the remaining network is from 1 – 126 total 126 networks.

Class B ( –

The default subnet mask for class b network is or /16. Class b network support supports the large networks up to 65,000 host addresses. The high-order bits for class b network is 10 in the first octet and the remaining bits of the first 2 octets create over 16,000 networks. The network is special network for link-local addresses, which also known as Automatic Private IP Addressing (APIPA).

Class C (  –

The default subnet mask for Class C network is or /24. Class C support small networks with a maximum of 254 hosts. The first three bits of the octet indicate the high order bit of the class. The remaining bits of the first three octets indicate the network and the 4th octet indicate host addresses in this class. The high-order bit is 110. Class C address has over 2 million possible networks.

Class D (  –

The first four bits of the first octet in Class D IP addresses are high order bits (HOB), the first four bits are 1110. The range of Class D addresses are start from to Class D is reserved for multicasting. In multicast communication data is destined for multiple hosts not for a particular host. The class has no subnet defined.

Class E (  –

The firs five bits of the first octet is reserved HOB for Class E address. The HOB for Class E is 11111. The address range is to This Class is reserved for experimental purposes only for R&D and Study. Like Class D, Class E is also not equipped with any subnet mask.

Public IP Addresses

A public IP address range is defined for network device, host, and servers like web server, email server to allow direct access to the Internet. Any server device using public IP addresses directly accessible from the Internet. A public IP address is globally unique, and can only assign once to any device in the world. Every device accessing the internet is using a unique IP address. Public IP addresses also required for any publicly accessible network hardware like the servers hosting a websites. Public addresses globally routed between different ISPs and routers. However, some addresses not routable on the Internet. These addresses called private addresses.

Private IP addresses

Private IPv4 addresses were introduced in 1990 because of the reduction of IPv4 addresses. The Private addresses are not unique and can be used again and again to internal network. The computers at home, tablets, smart phones, network printer; and the computers within organizations generally assigned private IP addresses. Computer with private IP address can see and access the local network through its private IP address.

The computer and devices with private IP address cannot directly access and communicate via the private IP address, however, using the router’s public IP addresses, the devices outside a private network can communicate. The NAT allows direct access to a local device assigned a private IP address. The range of private IP addresses are define for all three classes. /8 or to /12 or to /16 or to

Classless Addressing

Classful addressing divides an IP address into the Network portion and Host portions along octet boundaries. It uses a fixed subnet mask which is /8, /16 and /24, but, classless address uses a variable number of bits for the network and host portions of the address. The subnet mask is not fixed for classless addressing system.

The classful addressing system assigned 50% IPv4 addresses to Class A networks; 25% of IPv4 addresses to Class B, 12.5% of IPv4 addresses to Class C and the remaining 12.5 % Shared both Class D and E. The classful addressing plan waste the most of IP address which decreases the availability of IPv4 addresses. For example, an organization that had a network with more than 254 hosts would need a class B network with more than 65,000 addresses wasting 64,700 IP addresses.

To overcome the waste of IP addresses, classless addressing was introduced by IETF in 1993. In classless addressing system, there is no class of IP address, but the addresses has still granted in blocks. In classless addressing system, when an organization or individuals, need connectivity to the Internet; it also granted a block or range of addresses according to the need of organization and individuals.  For example an individual required only two addresses and an organization given thousands of addresses based on the number of its requirements.


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