New modem... IP changed and then went back to original?

I have Xfinity for internet and needed to change my IP address. The only way I know how to do this that actually works is by changing out the modem (in my case, the gateway). So I went and swapped out my old modem and got a different one. Got home and plugged everything in to make sure it was all good. My IP changed (checked a few different sites) and I was good for a day. The next day I checked again and it went back to the last IP I had with my original modem. Was I supposed to do something else in order for the new IP to stick? I've called Xfinity in the past and tried a few of their techniques to get the IP to change, but nothing ever works. The only way my IP has changed was with the new modem. But it seems that was only temporary. They did tell me that I have a dynamic IP, but I don't know how long the leases are.

5 Answers

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  • 2 months ago

    To change your IP address, turn off your modem AND your router. Leave it that way for a minimum of 3600 seconds.  But a day is more like the minimum. 

    Then turn the modem back on.  When the lights settle down, turn the router back on.

    The ISP will remember the association between your modem and the assigned IP address for that one hour minimum.  Then your old IP address goes on the stack of available addresses.  What needs to happen next is someone ELSE starts their router in your neighborhood without them having just turned it off. Then they'd get YOUR old address and you get the new one.  IF they had just turned off their modem, then their address would go on top of yours and they'd get that back. 

    Thus, my statement of a day is my figuring on the length of time it would probably take for someone to get your old IP address assigned to them.

  • ?
    Lv 7
    2 months ago

    I can't explain why the IP returned.  I can explain why leases don't matter as long as your connection is alive.  The DHCP server sometimes caches the MAC/IP mapping and when requested it will give the same IP back, thinking that is what makes most sense.

    The lease time doesn't matter because if your connection is alive, the lease on your modem/gateway expires, it asks and the DHCP server answers.  The server doesn't ask. you renew.  So the lease time is only how long you need to wait before the DHCP server expires it, but as I said unless it is reused by another it will give you back the same ip.

    BTW,  a cable modem/gateway DOES have a MAC and is registered.  The modem (typically) will only allow the first MAC seen to register, so you need to restart when using a separate router but not a router/modem.

    If you really want to change IP, I would request a static, pay it for a bit, then remove it.  I believe it is $20 / month for one IP.    They should be able to get you a "new" IP.

  • VP
    Lv 7
    2 months ago

    Why do you think you need to change your public IP address?

  • ?
    Lv 7
    2 months ago

    Press the Windows key + X.

    Click Network Connections.

    Right-click the connection that you want to change and click Properties.

    Note: The IP address for a wired connection is called Local Area Connection or Ethernet. The IP address for a wireless connection is called Wireless Network Connection or Wireless.

    Left-click Internet Protocol Version (TCP/IPv4) and click Properties.

    Select the Use the following IP address option.

    Enter your desired IP address and click in the Subnet Mask area, which should auto-complete.

    Click OK twice to apply the settings.

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  • 2 months ago

    Did you use the same router?  IP addresses are assigned to a MAC address.  The modem doesn't have a MAC address, the router that connects to the modem has the MAC address.  If the new device you used is a modem/router combo that would introduce a new MAC address, which in theory should solicit a different IP address.  

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