update fd-tap-ping.cc instructions

src/fd-net-device/examples/fd-tap-ping.cc

@@ -37,25 +37,25 @@
 //   |       1.2.3.4                 |     |
 //   +-------------------------------|-----+ 
 //                                   |
-//                                   |
-//                                   ------------ (Internet) -----                             
-//
+//                                   |                             +-------------+
+//                                   ------------ (Internet) ----- | Remote host |                            
+//                                                                 +-------------+
 //
 // To use this example:
 //  1) ns-3 will create the TAP device for you in the host machine.
 //     For this you need to provide the network address to allocate IP addresses
 //     for the TAP device and the ns-3 FdNetDevice.
 //
 //  2) Take into consideration that this experiment requires the host to be able
-//     to forward the traffic generated by the simulation to the Internet.
+//     to forward traffic generated by the simulation to the Internet.
 //     So for Linux systems, make sure to configure:
 //     # echo 1 > /proc/sys/net/ipv4/ip_forward
 //
-//     Also enable natting so the traffic sent on the Internet are able to 
-//     reach back the TAP. 
+//     Also enable natting so the ICMP replys from the remote host can reach 
+//     back the TAP. 
 //     - TAP-network-address is the same as 'tapNetwork'
-//     - net-device-ip is be the IP address of the network device connected to the internet
-//     # iptables -t nat -A POSTROUTING -s <TAP-network-addres>/24 -j SNAT --to-source <net-device-ip>
+//     - TAP-network-mask is the same as 'tapMask'
+//     # iptables -t nat -A POSTROUTING -s <TAP-network-address>/<TAP-network-mask> -j MASQUERADE
 //
 //  3) Before running the example make sure that the tap creator binary has root suid.
 //     If the --enable-sudo option was used to configure ns-3 with waf, then the following