We can use a server S, which has a permanent IP address, to test if a computer is located behind NATs or not. The server has two public IP addresses, i1 and i2, and has allocated two ports, p1 and p2 (either UDP or TCP port), for this purpose. Note that the IP address and the port number used by the server are known to every computer. (a) Explain how we can do the test by exchanging packets with the server. (b) There are many types of NATs. Assume we like to know if our machine is located behind a symmetric NAT or not. How do you design your test?