Hi Krishna,
You can have one network in project by changing "Network Assignment" to 1 (Project definition) in project profile - planning board/dates tab. This allows to create a network header at PD/root level WBS and from there on when you try to add new NWA under any WBS it automatically under same NH assigned to PD/root WBS
.
Note: Even with this setting system allows to create NH manually against the WBS.
Regards,
Pradeep