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Subnet / DSL Modem Question

By shdanfo
Mar 29, 2008
  1. About a year ago, with help from AT&T tech support, I set up a SpeedStream 4100 DSL modem to work with my linksys router. They had me connect my PC directly to the modem, reboot, and browse to to enter name and password information into the modem. After things were set up, I was able to connect my router's WAN port to the modem, and let my router assign 192.168.1.x addresses for PCs on my home network. But, even after doing this, I could still get to the modem at My net mask is, and I'm trying to understand why I can still get to the modem. I wouldn't think that would be on my subnet, but if my router is relaying the request to my gateway router, how would it know about my little DSL modem at Is the modem doing NAT for it's LAN address? Is that how it works?
  2. jobeard

    jobeard TS Ambassador Posts: 10,880   +903

    consider the wiring
    modem( 192.168.1.x)
    Because the modem and your WAN side of the router are in the upstream path of the
    AND the default routing out of the router is thru the WAN port, all traffic not intended for
    one of your LAN addresses will head for the modem and the ISP.
    If the modem sees its own address, then it responds. All quite normal tcp data flow.
  3. shdanfo

    shdanfo TS Rookie Topic Starter

    one more detail

    Thanks for your kind reply. I guess it's obvious. I thought that requests sent to a gateway for relaying on to the desired recipient (ultimately, on the correct subnet, and then using the MAC address of the recipient) were packaged within some sort of encapsulating routing request. And that the modem wouldn't be looking at such things.

    I guess if I really want to understand this stuff, I should spend some quality time with a good book on IP. (Something that, obviously, I've been putting off.)

    Thanks again. This is a great forum, and looking at people's questions and answers is fun.
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