Cisco IOS supports multiple IP Addresses on an interface. There will be one Primary IP Address on the interaface and multiple Secondary IP Addresses on the interface. There is no limit on the number of IP Addresses that can be configure on an interface. Configurng multiple IP Addresses on your cisco routers can sometime help where you have multiple subnets having one physical router interface.
The IP Addresses can be from different subnets and from different networks.
To configure multiple IP Addresses,
Configure primary IP Address
From the Interface configuration mode, configure the primary IP Address
ciscorouter# conf term
ciscorouter(config)# int fa0/0
ciscorouter(config-if)# ip address 172.20.20.1 255.255.255.0
That configures the primary IP Address.
Configure Secondary IP Address
Now, add the required secondary IP Addresses onto the same interface
ciscorouter(config-if)# ip address 172.20.21.1 255.255.255.0 secondary
This adds the additional IP Address onto the interface. In the command above, the keyword "secondary" at the end defines the additional IP Address as a secondary IP Address.