1) In Classful addressing it divides IP address into network ID and host ID

Class A :- has first octet as network ID and last three octet as Host ID

Class B :- has first two as network ID and last three two as Host ID

Class C :- has first three as network ID and last  octet as Host ID

2) Here class A,B and C are used where as class D is used for multicasting and class D for research

3) Disadvantage is that it limit the number of network that can be provided to the network

4) Example RIP(Routing Information Protocol) protocol uses classful addressing

5) Class A  :\8 as (1 octet is network ID) , Class B :-\16 , Class C : \24

6) Same Subnet mask is used in complete network

Classless Addressing :-

1) It allows us to use variable length subnet mask so also known as VLSM (Variable Length Subnet Mask)

2) Different subnet mask used in same network.

3) In this there is no boundary on host id and network id 

4) Classless Addressing also known as CIDR(classless interdomain routing)

5) There is no default subnet mask in classless routing.

6) Example: BGP(Border Gateway Protocol),RIPv2

for example Class A :- has first octet as network ID and last three octet as Host ID

Class B :- has first two as network ID and last three two as Host ID

Class C :- has first three as network ID and last  octet as Host ID

2) Here class A,B and C are used where as class D is used for multicasting and class D for research
While,
In Classless IP Addressing 
1) It allows us to use variable length subnet mask so also known as VLSM (Variable Length Subnet Mask)

Classful is based on the default Class A,B or C networks. 

 

- Class A: 0 - 127 with a mask of 255.0.0.0 (/8) 

 

- Class B: 128 - 191 with a mask of 255.255.0.0 (/16) 

 

- Class C: 192 - 223 with a mask of 255.255.255.0 (/24) 

 

Classless on the other hand, allows the use of variable length subnet masks, or VLSM, because subnet mask information is included with routing updates. You can have a mixture of different subnet masks in the same routing domain. 

 

10.1.0.0/19 

 

10.2.0.0/20 

 

172.16.8.0/21 

 

172.16.16.0/24 

2) Different subnet mask used in same network.

Class A begins with binary 0 (0-127.x.x.x)

So and IP address 0.0.0.0 to 127.255.255.255 use subnet mask = 255.0.0.0

Class B begins with binary 10 (128-191)

So IPaddresses 128.0.0.0 to 192.255.255.255 use subnet mask = 255.255.0.0

Class C begin with binary 110 (192-223)

So IP addresses 192.0.0.0 to 255.255.255.0 use a subnet mask = 255.255.255.0

 

Classless IP addressing means you can use any subnetmask you want, even assigning partial blocks. For example subnet 172.16.0.0 is a class B network.

But you have two physical interfaces on your router that connect to switches with 5 VLANs. Classless routing allows me to break up this IP address into more useful segments.

 Examples of classful routing protocols include RIPv1 and IGRP.

 

“classful” router has an interface connected to a major network, it believes it knows all routes connected to that major network. 

For example, a router may have an interface attached to the 10.1.5.0/24 network. It may also have routes from a routing protocol, also for the 10.x.x.x network. 

However, if the classful router receives a packet destined for a 10.x.x.x subnet that is not in the routing table, it will drop that packet, even if there is a default route.

- Class A: 0 - 127 with a mask of 255.0.0.0 (/8)

- Class B: 128 - 191 with a mask of 255.255.0.0 (/16)

- Class C: 192 - 223 with a mask of 255.255.255.0 (/24)

All devices in the same routing domain must use the same subnet mask. Since routers running a classful routing protocol do not include subnet mask information with routing updates, the router assumes either its own subnet mask, or defaults to the classful subnet mask.

 

Classless on the other hand, allows the use of variable length subnet masks, or VLSM, because subnet mask information is included with routing updates. You can have a mixture of different subnet masks in the same routing domain:

- 10.1.0.0/19

- 10.2.0.0/20

- 172.16.8.0/21

- 172.16.16.0/24

 

 

Classless:  This is really referring to the way routers treat the mask, and how they route packets.  In the old way, the subnet mask was not sent with the routing updates.  Now it is the way routers treat the mask when forwarding packets, (including routing updates)...the custom subnet mask is included.