Question

If a MAC address is unique, then why cannot we use MAC address instead of IP address to define a host in a network.

Networking, MAC address, IP addressing

If a MAC address is unique, then why cannot we use MAC address instead of IP address to define a host in a network.

Hello Bubai,
first the mac address is considered only at the physical layer while the IP address is at upper level.
Try thinking to a server where you mac address is unique like aa:bb:cc:dd:ee:ff and 2500 client computers know how to contact that server; suddenly your server dies because of a flooding and the mac address changes to aa:bb:cc:dd:ee:gg, how would you notice the client computers of this change? By using the IP address you simply remap the new server to that IP and client computer don't even know that there's been a hardware change.
I hope this clarifies an example of why the IP address is used instead of the mac address.

In addition we need the ability to mask to the IP address of systems through NATing for load balancing, and network security purposes.

There is also no way to ensure that every MAC address is unique through out the world.