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The difference between classful IP addressing and classless IP addressing is in selecting the number of bits used for the network ID portion of an IP address.

# In classful IP addressing, the network ID portion can take only the predefined number of bits 8, 16, or 24.

# In classless addressing, any number of bits can be assigned to the network ID.

To understand Classful vs Classless IP addresses, we first need to know what IP address classes are in the first place. IP address classes are as follows:

Class Range

A to
B to
C to
D to
E to

An IP address is broken down into a network part and a host part.

For example, the IP address is classified as class A because the first octet of 35 falls in the range of class A addresses. Class A addresses generally have the first octet as the network part and the next three octets as the host part giving us a default network mask of or /8

If this is the case the IP address is classful and originally all IP addresses were classful only. The shortage of IP addresses made way for classless addressing

Now consider the same IP address, which is a class A address and has the network mask /8. Imagine moving the network portion which is the first octet to cover the first two octets.

The network mask would still be or /8 but the subnet mask would be or /16 and we would be divided the network into sub-networks. One advantage is conservation of IP address space. Instead of assigning the whole address space to a client, we would assign the address space or even better the address space. The subnet mask would be determined by the required number of clients on the network.

Hence, we are freed from the limitation of using classful network boundaries like /8, /16, /24 as the demarkation for subnetworks, and can also have classless network boundaries like a subnet mask of /20 or

For example. Today, routers route both classful and classless protocols. Example of classful routing protocol is RIPv1. Classless routing protocols are EIGRP, OSPF, BGP are classless. Internally, since BGP comprises the backbone of the internet, the classful addresses are converted to classless and routed over the internet

So if everything is moving to classless addressing, where does classful addressing’s future lie. Well as far as the internet is concerned classless rules (however this discussion is not taking into consideration IPV6 where the limitation of IP addresses is not a problem.

For Lab Example:-

IP Classfull IP Classless = >

IP Classfull IP Classless = >

IP Classfull IP Classless = >

Understanding IP Addressing = > :

Class full IP Subnet Calculations = >

This Article Written Author By: Premakumar Thevathasan.CCNA, CCNP, CCIP, MCSA, MCSE, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+


Anonymous said...

This is very excellent way of teaching every one can be easy to understand this Article.

Anonymous said...

Classful vs Classless Addressing: