IBM RS/6000 - Manual (Page 188)

Figure 46. Network Topology Used in Our Lab
In the previous step, when you activated the NIM Master, a network object was
defined representing the network to which the NIM Master's primary interface is
connected. When this network is the only physical network you have, or when the
Master and the Clients are attached to the same network, you do not have to
define any additional network objects. If, however, the NIM Master and the NIM
Clients are not attached to the same physical network, as it is in our example, you
must define additional network objects.
To know whether or not a gateway must be defined to the NIM environment, take
the route that is used to communicate between the NIM Server and the NIM Client.
If, on this particular route, the gateway is immediately connected to the network
containing either the NIM Master or NIM Client, then this gateway must be defined
to the NIM environment.
164 Introduction to PCI-Based RS/6000 Servers