Comcast dynamically-assigned IP addresses or statically-assigned IP addresses

Posted on 2006-03-22
Last Modified: 2008-02-26
    I heard that Comcast uses dynamically-assigned IP addresses for home internet accounts, but when I check my IP address by using ipconfig from the command line or going to a site like my IP address seems to always be the same.  Is there a way to change your IP address each day or each time you connect to the web?  I did use a hardware router but now I connect directly to my cable modem.
Question by:gdkinney_2
    LVL 3

    Accepted Solution

    With cable you usually keep the same IP address for sometimes months at a time...The only way to change it would be to powerdown the modem for sometime and hope for the best....If your concerned about security I would suggest hooking the router back up as this uses NAT and makes your computer invisable to the internet in a sense.
    LVL 15

    Assisted Solution

    When you power up your cable modem, one of the things it does is request an IP lease from Comcasts DHCP server. Comcast finds an available IP address to assign to you and you lease that IP. Typical IP leases can be between 8 hours for up to 2 or 3 days. When your lease is up, your modem renews the lease for that IP. If you want to change your IP, leave your cable modem unplugged for over a day (typical leases are 24 hours). When you replug it in, it will ask for a new lease and most likely the IP will be different. You could try to release and renew the IP. For XP, go to a command prompt and type "ipconfig /release" then press enter. You can then type "ipconfig /renew" then press enter. This release and renew will clear the lease and should assign another IP, but the DHCP server may not accept it. Unplugging as described earlier should work if that doesn't.

    I would highly suggest the router be reimplimented. If you really do not wish to use it, make sure you have bullitt proof security setup and you will be prone to alot of port scanning.

    Hope this helps
    LVL 1

    Expert Comment

    If you truly want a new IP, I would power off your cable modem and wait a day or so before you power it back on. That would be your best bet at getting a completely new IP.

    The only other way to protect  your network is to use a hardware-based firewall and NAT. Then put your PC on a private IP range (10.x.x.x, 172.16.x.x, or 192.168.x.x).
    LVL 12

    Assisted Solution

    actually you want to look at the lease time by doing


    this will tell you how long the mac address of your device will be stored in the ISP DHCP server ... you normally receive the same IP address as long as the pool of addresses is not running low. You can either shut off your device that is connecting to the modem for the lease period, OR you can put a different device with different MAC address.. if your router is capable of modifying its MAC address, you can do that too...

    LVL 12

    Expert Comment

    1 more note-- your DHCP client will automatically try to RENEW the lease at 50% of the lease time
    LVL 2

    Expert Comment

    This is a hit or miss question.  It really depends on many factors.  RCN cable in Massachusetts is a former ISP that I have used, I had the same IP address for 2 and a 1/2 years under their "dynamic" addressing!

    For the last six months I have used Comcast, it has changed once, and it was during the first week I had the service.  Since then it has been the same.

    As far as I understand it, this is completely under the control of the ISP, and the address is assigned to your modem, so any changes on your internal setup would have no impact at all.
    LVL 12

    Expert Comment

    I used to work for them back 5+ years ago and that was the case in my area when they started cablemodems.. few years ago they switched to DHCP.. so I guess it depends on Area.. the way to tell is if you can get on with a DIFFERENT device your good with DHCP... if not its a programmed MAC
    LVL 18

    Assisted Solution

    you will always get the same IP after hardware reboots if the pool of IP's is not over used and the IP you had 'last time' is still available.   To get a new IP you have to change your MAC address, which is very easy on most home routers because you can manually enter one.  The problem is then COMCAST usually uses your MAC address to authenticate you on thier network, so changing frequently might not work.

    For anonimity try some other method like proxies.

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