Physical isolation means that the intranet is not directly or indirectly connected to the public network. Physically isolate the internal and external networks to ensure that the external network cannot invade the internal network through network connection; At the same time, the information in the internal network is prevented from leaking to the external network through network connection. Physically isolate the internal network from the external network to ensure that the internal network information will not be leaked to the external network through electromagnetic radiation or coupling. The two network environments are separated from physical storage, and the components that will lose information after power failure, such as memory, processor and other temporary storage components, should be removed during network conversion to prevent residual information from leaving the network; For non-volatile devices without power supply, such as tape drives, hard disks and other storage devices, the internal network and external network should be stored separately.