Why is my ADSL modem not listed as a gateway (chaining two routers)?

Question

I just got a new Wifi Router and connected it to my ADSL modem. The Wifi router assigns IPs in the range 192.168.0.xxx and the ADSL modem assigns IPs in the range 192.168.1.xxx.

The ADSL modem is configured using bridge mode, while the Wifi router is configured using PPPoE mode. I have only a basic understanding of networks etc, so please bear with me. :)

When I run traceroute -n google.com, only my Wifi router is listed as a gateway, the ADSL modem is not. Here's the complete output.

 1  192.168.0.1  2.273 ms  3.508 ms  3.500 ms --------->(Wifi router IP)
 2  117.205.xx.1  61.515 ms  63.447 ms  65.360 ms ----->(ISP gateway?)
 3  218.248.174.246  71.333 ms  73.326 ms  73.501 ms
 4  115.114.57.249  117.321 ms  119.263 ms  121.230 ms
 .....

Also, my routing table doesn't list the ADSL modem either. Here's the output of route -n.

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.0.1     0.0.0.0         UG    0      0        0 ra0
169.254.0.0     0.0.0.0         255.255.0.0     U     1000   0        0 ra0
192.168.0.0     0.0.0.0         255.255.255.0   U     2      0        0 ra0

Can someone explain me the reason behind that? Why is my ADSL modem is being invisible? It's obviously sitting there and routing packets. When I connect to the ADSL modem directly using ethernet, I have to set up port forwarding on the modem. But when I connect to the Wifi router, I don't have to set up port forwarding on both the devices - just the Wifi router and it works. Can someone explain me the concept? Thanks a lot!

Answer 1

If the modem is in bridge mode, it's not actually routing the packets, at least not in the sense of it modifying them (NAT). Bridging is designed to transparently/seamlessly connect two networks together as if they were one, large network, which is the opposite of a gateway which might be doing NAT.

By having the ADSL modem in bridge mode, it is effectively transparent to the network, unless you're trying to communicate with it directly (for example, by trying to point your webbrowser at it's IP to configure it). It's just tunneling your packets to the ISP.