IP by MAC address
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Thread: IP by MAC address

  1. #1

    IP by MAC address

    I just moved and I've been too lazy to take my router out of it's box.
    So I've been manually disconnecting and reconnecting the CAT5 from computer to computer.
    While doing this, I realized my ISP would assign me a different IP

    I've always thought I'd have to call them up, to request a different IP.

    By theory, shouldn't I be able to spoof my MAC address various times, to get assigned different IPs?

    Or, are there other factors to take into account of here besides my MAC address?

  2. #2
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    The MAC and IP are tied together for the duration of the IP lease, or longer if the connection remains in tact...


    After the lease has expired, the IP will be reassigned.


    There is no correlation between MAC and assigned IP unless a specific filter is in place on the DHCP server that assigns IP based on MACs

  3. #3
    Senior Member IKnowNot's Avatar
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    What MAC address is the ISP seeing ??
    " And maddest of all, to see life as it is and not as it should be" --Miguel Cervantes

  4. #4
    Senior Member nihil's Avatar
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    OK guys, just have a laugh if you want

    Over here I have a single ISP connection, that I use with several boxes................one at a time!!!

    So each time I connect, I get a different address.............the ISP sees the MAC addy of the box I have used, but are not really interested................they are looking at my telephone account as that is how they will bill me.................

    I guess you should be thinking from the top down?


  5. #5
    Master-Jedi-Pimps0r & Moderator thehorse13's Avatar
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    By theory, shouldn't I be able to spoof my MAC address various times, to get assigned different IPs?
    The short answer is no.

    First, some history.

    Some ISPs used to dump the MAC address in their DHCP scope and use that as part of the DHCP lease process. This was done to stop people from adding more than one host to their network or so the story goes. Other than the fact that this is an utterly idiotic and easily beaten process, it also takes a tremendous amount of administrative time from the provider. Whenever a NIC went south or you changed it out (even for testing and such) you had to call their support line.

    Most SOHO routers (LinkSys for sure) have provisions for MAC spoofing built into the device. I can't imagine that many ISPs are still adding MAC addys to their DHCP scopes. They have bigger fish to fry these days.

    And now, the details to the answer.

    Windows hosts will always try to get the same IP address from a DHCP server, even after the lease has expired and your NIC/MAC has changed. Only if the address is already taken, will it move on and grab a new one. The IP address is held in the registry and it will always go after it unless it has been reassigned regardless of MAC or otherwise.

    After the lease has expired, the IP will be reassigned.
    Let's tweak this d0pDizzle. After the lease expires, the IP *can* be reassigned.
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  6. #6
    Shrekkie Reloaded Raiden's Avatar
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    Most SOHO routers (LinkSys for sure) have provisions for MAC spoofing built into the device. I can't imagine that many ISPs are still adding MAC addys to their DHCP scopes. They have bigger fish to fry these days.
    Sadly there still are some yes. One belgian operator STILL takes advantage of unexperienced users and sell them subscriptions where they have to pay monthly for 1, 2, 3 or more boxes ...
    This is exactly easily beaten by most users with a broadband router, yet still people (mostly the older folks) get screwed like this. Sad indeed.

    My 2 eurocents,

  7. #7
    First off, thanks d0pp and Horse for providing some interesting information.
    Darn, I guess no "ipconfig /release", "ipconfig '/renew" is going to work on a Windows box with this one.

    Any of ya happen to know when a lease "knows" to expire?
    I don't think I've ever understood this process too clearly.

    Over here I have a single ISP connection, that I use with several boxes................one at a time!!!

    So each time I connect, I get a different address.............the ISP sees the MAC addy of the box I have used, but are not really interested................they are looking at my telephone account as that is how they will bill me.................
    Wow, that certainly caught my attention.
    Are you recieving phone service from your ISP too?!

  8. #8
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    Hey Hey,

    You have to look at the various camps before getting into a discussion regarding ISP IP Addresses.

    Dial-up: With 99% of these accounts you get a different IP every time you connect... and it doesn't matter where you connect from... (I say 99% because I still know people using "Business Dialup")

    DSL: These accounts will generally assign you a new IP every time you connect... (Unless you have a Static IP with a Business Account)... This is most likely what nihil is talking about... The majority of DSL accounts use PPPoE....However there are some (Verizon comes to mind that have a seperate service.... They just call it DHCP.... Big with Verizon West I believe it is... but that's changing now)... With the DHCP Customers.. they're connected and they get an IP... If there's a valid lease... then they get that IP... if there isn't... They get a new one... with PPPoE it'll change everytime... While it's still a dynamically assigned addess it's different than just being on a network and geting an address from the DHCP server... the IP is sent in an IPCP packet with most PPP related processes..

    Cable: Here is where many cable companies (and as Raiden pointed out some still do) would put on Mac filtering... When you first connected, your Mac address would be remembered... You could then only connect from that Mac.... now most people with cable have Dynamic IPs (assigned by DHCP)...yet they never notice their IP change... This is because the lease hasn't expired... or the company has set it up that way.... When you shut down Windows (If I remember correctly) it doesn't release it's current lease.... that lease will still be there when you start back up next time.. So you'll get the same IP... when you have a router, you generally never turn it off so just before the address expires (depending on the OS) a packet will exchange will take place to ensure that the device continues to use that address and to expand the lease time.. Some cable companies that I've used have actually assigned you an IP based on your modem.. regardless of what you do... that IP is yours.

    With that said.. you have to understand DHCP to get how this is all working...

    You can find a tutorial that describes the basics of DHCP @
    http://www.antionline.com/showthread...hreadid=258242

    If you want to see an example of PPP in operation (used in PPTP and not PPPoE but it will give you an example) you can go here:
    http://www.antionline.com/history/to.../269784-1.html

    Peace,
    HT
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  9. #9
    Just Another Geek
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    Originally posted here by ThePastorGang
    Any of ya happen to know when a lease "knows" to expire?
    ipconfig /all should tell you when the lease expires.. When the client recieves an IP address it also recieves a lease time. At half that time the client will ask the server if it's still ok to use the IP address. If the server doesn't respond it will keep using the IP and ask again at 3/4 of the lease time. If the server still doesn't respond the client will ask one more time at the end of the lease time. If the server still doesn't respond it will loose it's IP. At the 1/2 and 3/4 and 1/1 time the server can respond with an OK or with another IP. Then the lease period (and this whole process) will start over again..
    I don't think I've ever understood this process too clearly.
    It's all part of DHCP..
    http://en.wikipedia.org/wiki/Dynamic...ation_Protocol
    Oliver's Law:
    Experience is something you don't get until just after you need it.

  10. #10
    Senior Member nihil's Avatar
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    Wow, that certainly caught my attention. Are you recieving phone service from your ISP too?!
    Yes, I am...............it works out a lot cheaper over here as a multiple deal.............some outfits (Toucan?) are even offering mobile phone in the deal as well. At the moment my ISP/TELCO owe me around $90(USD)...........I guess that is probably due to some beta tester's rebate, but it is nice to have them owe you?

    HTRegz is spot on, I was talking ADSL (SDSL is pretty pricy and only used by banks, brokers and the like over here)

    We also have static addys, but you have to pay extra for them.............once again a business requirement. I have never tried them, but what the other guys are talking about sounds like our cable/satellite stuff?




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