IP Address: I thought I understood - maybe I don't!

Discussion in 'other software & services' started by LockBox, Sep 29, 2009.

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  1. LockBox

    LockBox Registered Member

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    Sorry if this is a little long. But something really strange has happened.

    I've always thought I had a very good understanding of IP addresses and how they are assigned, static vs DHCP, internal/external, etc. However, I am completely baffled by what I am about to write and maybe I am about to find out I didn't know as much as I thought I did.

    Whenever I visit any "what is my IP" type website it always tells me that my IP is (the following is fake for privacy purposes): 63.226.200.107

    It's been that for about 9 months now. Fine, my IP address is 63.226.200.107.

    Fast forward to tonight when a family friend needed Internet access at their house but their router was down and they could not connect with a direct ethernet connection. I know that can sometimes be tough and time consuming so I offered to bring my Linksys router over and let him connect at home and get his printing done, etc.

    I took my laptop over to his house, along with my router, as I had things I wanted to do as well. Out of curiosity, I went to the "Check My IP" site I usually use and surprise - it was the exact same IP as the one I always see at those sites: 63.226.200.107. I went to the a couple of other sites like Still Listener that shows more data and it still insisted the Ip was 63.226.200.107. Hey! That's mine!

    So, what's going on? I thought the IP above was my public IP and my router then assigned other network cards numbers like 192.168.1.111, etc. But here, after my computer was cleaned out by CCleaner, rebooted, everything you can think of, and it still showed my friend's IP, using my router, as the IP that I thought was MINE: 63.226.200.107.

    Can somebody explain? How do these sites see MY public IP as HIS public IP on a completely different set of pipes 15 miles away from my home address?

    Does that mean my router, as long as I use this one, is always going to publicly identify me with that number - no matter what? What if I moved? My friend's house could have just as well been a new house of mine and there I would have had the same public IP!

    Consider me completely confused. Can anyone make sense of this? As always, thanks in advance!
     
  2. noone_particular

    noone_particular Registered Member

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    If your friend has the same ISP, it's possible. Some ISPs have their customers on networks that are much like a LAN where a group of customers are all on the equivalent of an ISP owned LAN. The customers IPs are assigned by DHCP and have a fairly long duration. The IP is programmed/stored in the modem. When you hooked it up at your friends place, it's still connecting to the same vendor network "LAN equivalent" using the same IP. Is this Charter by any chance?
     
  3. I no more

    I no more Registered Member

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    So, I assume you took just the router and not the modem. Like noone_particular, I'm also thinking your friend has the same internet provider as you. I will also assume that you didn't change the MAC address of the router when you took it to your friend's place.

    If all of the above are correct, could you try changing the MAC address of the router, then check the IP at your place? If the IP changes, then try the router with that same MAC address at your friend's place. I guess what I'm wondering is if your ISP is assigning the IP based on the MAC address of the connecting device, perhaps regardless of which residence it's connected in.

    I know for a fact that anytime I change the MAC address of my connecting device, I get a new IP. I use this sometimes when sites like rapidshare incorrectly tell me that my IP is already downloading a file.

    But make sure to return to your original MAC when you're done with the experiment. Sometimes, the ISP will provide service contingent on you maintaining the same MAC address you initially registered with. Otherwise, you might find your service not working. I know of one ISP that allowed a different MAC address to work for 2 weeks before they cut off service.
     
  4. Fly

    Fly Registered Member

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    Your ISP probably assigns IP addresses to MAC addresses. That's not uncommon. You did hook up your laptop to your router ? An ISP can also assign an IP to the ethernet card in your laptop.

    I wouldn't recommend changing your MAC address unless you need to and know what you are doing. I once did that and got into trouble. I tried resetting the router, but the reset probably didn't reset the MAC.

    That's probably how I lost my previous router.
     
  5. Mrkvonic

    Mrkvonic Linux Systems Expert

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    ISP can assign IPs based on user trying to authenticate.

    I have one such account where I get a unique IP regardless of which machine I connect from. The IP address is given on the user & password submitted during the dial procedure.

    Mrk
     
  6. YeOldeStonecat

    YeOldeStonecat Registered Member

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    Sites like WhatIsMyIP or CheckMyIP or StilListner you link will show your public IP address, not your computers private Class C IP addresses from behind a NAT box.

    So say you have 10x computers behind a NAT router sharing a single public IP address, and they all go do whatismyip.com you'll see the same public IP address posted from all 10x computers.

    What was mentioned above about the MAC address of your routers WAN port is probably the case here, you're probably on cable internet, and since your neighbor is on your local node, the same MAC address of your router asking for an IP results in the same IP address being given until the lease is up. That's nothing to worry about. Bring in a different router and you'll probably get a different IP address.
     
  7. I no more

    I no more Registered Member

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    Another way to change the MAC address of the device connecting to the cable modem is to connect directly to the modem with your computer (i.e. ditch the router). If the MAC address of your network card is different than your router, then you've accomplished the same thing. But it's also possible your router cloned your computer MAC address. So, your computer and router may have the same MAC.

    But, if that's the case, it's also pretty easy to change the MAC address of your network card. I wouldn't try to change the routers MAC address unless the router's software has a direct way to do it. With most new routers, when you go to the setup, it will have a way to easily change the MAC. In fact, most new routers will allow you to directly clone your computer's MAC. Just make sure to write down the previous MAC before changing anything.

    And, I would follow the advice above about not doing anything unless you're sure you can reverse it.
     
    Last edited: Sep 29, 2009
  8. Fly

    Fly Registered Member

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    About changing the router's MAC:

    In my router could change or 'clone' the MAC. It seemed easy.

    I was pretty sure I could reverse it ... Wrong ...

    Certain behaviours of my ISP may also have been a factor.
     
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