I need to change my IP address.

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@shmrljo007 wrote:

Have to change my ip adress for security reasons but trying the "ipconfig/renew" command prompt method doesn't work.

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Which IP address are you referring to?  The public address the Comcast network assigns to your modem/gateway?  The 'ipconfig/renew' option only 'renews' the existing address for another 24 hour period.  You might try the 'ipconfig/release' command to get a new address, however for a PC/laptop/device, that only changes the address on your local network, usually a private address.

You might look at https://forums.xfinity.com/t5/Your-Home-Network/Router-External-IP-Address-Port-Forwarding/m-p/3290879/highlight/true#M312673

That thread suggests powering down the modem and router for 1 minute, and powering up the modem first, fully synced, then the router.

Might not be your exact scenario though.

Yeah the public address

Powercycling the modem won't result in a change of the public IP address. Neither will performing an IP release / renew if one is using a router. That would only release and renew the LAN / private address...

OP. If you are using a router that has a MAC address cloning feature, try cloning a different MAC address in to it. This will force their DHCP server to issue a different public IP address to your connection. 

I'm not sure where to do that though. theres no option on my gateway settings to clone the MAC address

The Comcast supplied gateway devices do not offer a MAC address cloning feature.