Please read it carefully In first paragraph only first line describes classful addressing remaining is all about classless addressing if u want example
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.
Switch 1 Switch 2
VLAN1 172.16.1.0/24 VLAN10 172.16.10.0/24
VLAN2 172.16.2.0/24 VLAN11 172.16.11.0/24
VLAN3 172.16.3.0/24 VLAN12 172.16.12.0/24
VLAN4 172.16.4.0/24 VLAN13 172.16.13.0/24
VLAN5 172.16.5.0/24 VLAN14 172.16.14.0/24
So I have made 10 useful subnets from a single network. Here's the real advantage:
1. When my router is configured for classless routing it can advertise all 10 networks as one IP address 172.16.0.0 255.255.240.0, or I can advertise each physical interface using 172.16.0.0 and 172.16.8.0 using ths subnet mask 255.255.248.0
Classless networks enable the use of IP addresses with different subnet masks in the same network to enable efficient use of IP addresses