[mogmismo]

Thanks. For future travellers on this path:

sysconfig

option 5 - Network Connections
option 1 - Add new connection
option 1 - Secondary IP on interface (alias)
option X - ethX, which one you want to add the connection on
IP Address - Enter IP
Netmask - Enter Netmask
Broadcast - Enter Broadcast (or enter for default)

Done.

Also, and it confused me, the interface must be UP for the command to work. It will not work if the interface is in a down state.