I currently pay for a package with Xfinity. I am getting cable & internet into my place (residential).

Recently, I decided to start studying for a CCNA R&S. So I picked up a lab, a Dell poweredge, and a rack from Amazon. I thought, "I would like to put this on a separate network than what my home network is using," so I bought a new modem too (plus a coaxial splitter). I get all my stuff, hook up the modem, decided to test to make sure I was getting internet to it. I wasn't.

A quick search on the internet (with the working modem) shows that

  1. I need to contact Comcast to get a second account set up (and subsequently, a second modem...)
  2. I would have a second IP (which I don't care one way or the other)

So the question comes in: What is another way that I would be able to set up what I am trying to achieve?
Idealy, I don't want to "tell" Comcast's modem about this second network (can you even set up such a thing with what they provide?). I would like for it to stay two completely separate networks.
I REALLY don't want to give comcast more money (so a second bill/plan is basically out of the question, unless there is literally, no other way)

The main goal here, is to be able to connect (and learn, obviously) my cisco equipment, connect to the servers on this second network... But still utilize things like my ASA when I practice setting up firewall rules.

To my knowledge, routing everything through the comcast modem would be considered 1 big LAN and firewalls are pretty lax when it comes to lan-to-lan traffic. So my ASA wont work as expected. Correct?

Am I just over thinking all of this?

Relevant stuff:

  • Comcast modem - Cisco DPC3941T
  • New modem - CM500
  • ASA 5505
  • Why not just connect everything on the stud network in its own IP range and use the same network as you already have. That probably solves 90% of all you need without requiring extra hardware. If you want to play fancy get a second router (not a modem) and let that play default gateway for your test network. (connect one port from that routes to your home network if you want to reach the Internet and configure accordingly).
    – Hennes
    Feb 16, 2016 at 7:51

2 Answers 2


It looks like a coaxial splitter is not the correct way to join the new hardware to the old network, and you need to use something else instead. I'd expect a cable splitter to be suitable for connections that only divide an incoming signal, but internet connections have signals in both directions, and the modems are rarely designed to watch for a signal from a different modem when deciding what times they should wait before transmitting anything.

Using a router connected to the old modem is one possibility. Or, if there are enough extra ports on the old modem, connect the new hardware there, but don't let the IP addresses assigned to the new hardware be known to the old hardware other than the modem.

  • Sorry for the long delay in responses. Have been busy at work and havent had much time to play with my new toys. After thinking about it for a little longer, I def over thought the entire situation. This is more or less what I am planning.
    – Oberst
    Feb 19, 2016 at 21:44

While it typically is a VERY bad idea - you'd want to look at is double NAT. Just treat your main router like you would your second modem - set up a different IP address range for the network from the cisco out, maybe mess with routing from your main lan to the new one and bob's your uncle.

You cannot use a co-axial splitter like that. Its meant to so you can use one outlet for internet and phone, and your ISP ties your internet connection to the mac address of the new modem.

  • I realized that I couldnt just plug a new modem in after doing a little research. Never occurred to me that I would need to involve my ISP to achieve what I was trying. Can you elaborate on necessarily WHY it is a bad idea? Does it just complicate the troubleshooting? Is it less secure as a whole?
    – Oberst
    Feb 19, 2016 at 21:50
  • It's just messy and complicates routing. Something to avoid usually cause it's messy but for your needs seems perfect.
    – Journeyman Geek
    Feb 19, 2016 at 22:33
  • Ah. I see. You're right. In this scenario, I dont think it would be too terrible if I used a messy solution this time. Thanks
    – Oberst
    Feb 19, 2016 at 22:39

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