Thanks ejs, the reason I'm asking is because I found a tutorial on how to access the modem GUI if you are behind a router (I have a Draytek in bridge mode and an Asus RT-AC66U_B1 as router) and I found the following two commands (the modem IP address is 192.168.1.1):
ifconfig $(nvram get wan0_ifname):0 192.168.1.2 netmask 255.255.255.0
iptables -t nat -I POSTROUTING -o $(nvram get wan0_ifname) -j MASQUERADE
Now, with just the first command I am able to access the GUI, therefore my question as I thought that if not using the second command I could have had some security issues... but by reading your answer, I understand I'm safe also not running that iptables command, am I right ?