When you configure the specified external port numbers of the NAT server, if the carrier has disabled the port numbers, the NAT server is unavailable. Therefore, you are advised to use non-well-known port numbers as external port numbers when you configure the NAT server.
The troubleshooting procedure for a failure to make the NAT server function take effect is as follows:
Use the display nat session all command to check whether NAT entries exist.
If no entry is found, check whether the mapped public IP addresses can be used and the external port numbers are disabled.
If entries are found, check whether there are error statistics and obtain packets on interfaces and analyze them.