If no public IP address was assigned when you purchased the CVM, you cannot re-apply for a common public IP address for this CVM. However, you can achieve this purpose by using EIPs. For details on how to apply for EIPs, see Applying for EIPs.
An instance without a public IP address can apply for an EIP (see the previous question) or can access the Internet through the NAT gateway.
A NAT gateway provides CVMs in a VPC with the SNAT and DNAT features. If you have multiple CVMs that need to access the Internet through a public IP address, you can use a NAT gateway.
Yes, you can.
After you convert a public IP address into an EIP, we recommend that you immediately release the EIP. Otherwise, the EIP that is not bound to any instance will incur resource occupation fees.
Due to the limits on EIP resources, each account can apply for up to 20 EIPs in each region, and this quota cannot be increased. CVMs without public IP addresses can access the Internet through a NAT gateway or other means.
If a CVM has a public IP address or EIP and its subnet is associated with a NAT gateway, the route table specifies that the next hop for the traffic of this subnet to access the Internet is a NAT gateway. In this case, all the traffic of this CVM to access the Internet flows through the NAT gateway by default.
If you need to modify the priorities to redirect the traffic of the CVM to access the Internet through a public IP address, see Adjusting the Priorities of NAT Gateways and Public IP Addresses.
No, the network fees will be charged only once. When you access the Internet through a public gateway or NAT gateway, only the network fee for using the public gateway or NAT gateway will be charged.