IP address mix up

  • Posted: 24 March 2011 09:33 AM

    Hi there,

    I’m using DSL with a Fritz!Box right now and the following ip setup: Fritz!Box AppleExtreme
    10.0.1.x several Macs and other devices is used as my router.

    Today a cable modem (Arris) was installed and I will switch over to cable soon. I tried to change the ip of the cable modem to but had no luck. It only came with windows software and the web interface I found running on the cable modem would not let me change the ip address. The cable modem seems to have as ip address.

    I only have little knowledge of networks. So here is my question.

    Would it be wise to change all my devices to the 192.168.100.x subnet and set as the new router?

    Or is there a way to leave my net on 10.0.1.x and somehow get the Fritz!Box to “route” everything to the cable modem? Or better use the AppleExtreme as router from now on (since after the switch the Fritz!Box will only be used as a telephone switch board)?

    Any help would be appreciated!

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    Posted: 30 March 2011 08:29 PM #1

    To be clear, the cable modem is apparently a modem and router combined. If it was just a modem, you wouldn’t have this issue; you’d just use your own router.

    You can certainly leave your net as is, and route through the Fritz|Box to the Arris (just set the WAN IP address to with .1 as the gateway). My experience with doing double-NAT like that, the few times I’ve tried, is that it’s slow.

    (NAT: Network Address Translation: ability for all your devices to be seen on the internet as coming from just one address). Double-NAT is just doing NAT one extra unnecessary time.

    Seems to me that you have the following options (in order of preference):

    Reconfigure the Arris (maybe through the web interface if possible, or by borrowing a Windows machine) to be in bridge mode. When it’s in bridge mode, it’s own IP address is irrelevant, and you can configure whichever other router you want to use as your router. You’ll still be going through both your router and the Arris, but it’ll be faster than if it’s doing NAT twice.

    Change all your internal addresses to be in the 192.168.100.x range and use the Arris as your router.

    Borrow a windows machine and reconfigure the Arris’s IP address to

    Do that double-NAT with either the Fritz|Box or the AppleExtreme, and take the possible performance hit.

    I’d prefer the first because it gives you the most level of control. I don’t know how much other control you get with the web interface (ability to open ports, etc), but having to borrow someone’s laptop every time you want to reconfigure your router is going to get old fast.

  • Posted: 31 March 2011 06:45 AM #2

    Thanks a lot for the elaborate explanation.

    I initially also thought that the Arris (722) is a modem and router. But as I couldn’t get it to work I found out that is only a “stupid” modem.

    I cannot change the IP of the Arris. With the web interface you can’t change any settings. You need a “password of the day” that will allow access to a service menu, but that is only for technicians. And the Windows setup software only handles the connection from the PC to the modem but won’t allow to change settings.

    So I invested some hours work an set all my devices to the 192.168.100.x range. Now I’m on the save side and it works.

    Thanks for your thoughts and showing me what alternatives there are!

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    Posted: 31 March 2011 02:37 PM #3

    Well, I won’t argue the point that it’s stupid. But if it’s talking to several computers and they all are in the 192.168.100.x range, then it’s both a stupid modem and a stupid router!