It depends on networking mode you choose. If it is system/static mode (network is not explicitly managed by Eucalyptus), then they can be on any subnet as long as they can communicate each other. If it is managed mode, NC, CC, and SC should be in the same subnet because they form a virtual cluster. Still Walrus, CLC can be on different subnets and talk to multiple virtual clusters.

-SM

You'd have one CC and SC on each network. CLC and Walrus can be anywhere. For more information see the network configuration guide:

http://open.eucalyptus.com/wiki/EucalyptusNetworkConfiguration_v2.0