What is the command to add another IP address to an interface that already has (at least) one IP
address?

A.
ifconfig eth0:1 192.168.1.2

B.
ifconfig eth0 192.168.1.2

C.
ipconfig eth0:1 192.168.1.2

D.
ipconfig eth0 192.168.1.2

E.
ifconfig eth0:sub1 192.168.1.2

Explanation:
From the man pages:
ifconfig – configure a network interface
The interface is usually a driver name followed by a unit number, for example eth0 for the first
Ethernet interface. If your kernel supports alias interfaces, you can specify them with eth0:0 for the
first alias of eth0.
You can use them to assign a second address.