For how much period does the NAT table entry stays there even after connection is closed?

Question

Yesterday, I was reading about the NAT then, during that time I endup with above doubt, I am thinking that if some application from private network makes call to outside world i.e. public network using some XYZ port,

Now, This entry will be stored in the NAT table, If I get the external port and external ip for that application request then, Is it possible for me to establish the some sort of connection from outside public network with the host in the private network by closing the connection which he/she made using XYZ port and immediately binding to my application with XYZ port?

If it is not possible then, how can i connect to my application running in the private network from outside the private network?(tunnelling?)

Answer 1

The period the table entry remains open depends on the router model, software version, and configuration. For TCP (connection-oriented), many will keep the port open as long as the TCP connection is up, or for a configured time period with no activity. For UDP (connectionless), it will be for a configured time period. The time periods are vendor dependent.

The NAT table entry will have the source and destination IP addresses in the table. Unless your outside host has an IP address of a table entry, you will not be able to originate a connection from outside.

You cannot originate a connection from outside unless you have some way to bypass the NAT, e.g. a tunnel, or you have configured static entries for the NAT.