What is the concept of virtual NICs and in which cases it is useful ?

Question

One server may have support of limited number of physical interfaces. What if there is need of more Ethernet interfaces but physical slots are not available ?

Answer 1

A system always have limited number of physical NICs. Each NIC will have only one IP address. In case we need multiple IPs in same machine we can virtual interfaces such eth0:1 , eth0:2 etc but all of them will be having same series IPs as per my understanding.
In case if your machine want to be part of different LAN then you have to create interface with VLAN ID.