On default you should not worry about the subnet id and subnet mask. The original IP address detection routines will cause GraphLab to try to use the first IP address of the machine which is not localhost.
However, this is an issue for systems with multiple NICs, where for instance, one NIC is used for external networks, and one for internal networks. Selecting the wrong address will prevent GraphLab from operating correctly.
Starting from GraphLab 2.1, GraphLab has the capability to select the network to use.
The IP detection routines will read the environment variables GRAPHLABSUBNETID and GRAPHLABSUBNETMASK if available, and will use the first local IP address matching the subnet.
mpiexec -n 2 env GRAPHLABSUBNETID=192.168.0.0 GRAPHLABSUBNETMASK=255.255.0.0 ./pagerank ....
will have pagerank run over the 192.168.0.0/255.255.0.0 network.
If only GRAPHLABSUBNETID is provided, it will try to guess the subnet mask by left extending the subnet ID.
If both GRAPHLABSUBNETID and GRAPHLABSUBNETMASK are not provided, they default to 0.0.0.0 and 0.0.0.0 which corresponds to the original behavior.
You define the environment variable using either export (on bash) or setenv (in tcsh) command based on your linux version.