If a secure network has a few devices, such as a campus network or a small enterprise network, SNMPv1 can be deployed for the normal communication between the NMS and managed devices.
If your network is in a large scale with many devices and its security requirements are not strict or its security is good (for example, a VPN) but network congestion may occur, SNMPv2c can be deployed for communication between the NMS and managed devices.
If your network has a strict requirement on security, only authorized administrators can manage network devices, and the security and accuracy of transmitted network data need to be ensured. In this case, SNMPv3 can be deployed to ensure the normal communication between the NMS and managed devices.