Got to be careful with that statement. If your ISP dynamically assigns addresses to its customers, majority of the time, you have an ip address for a specified period of time (leased) decided by the ISP. Yes, you might be able to force the issue with a ipconfig /release /renew ... but highly unlikely. It usually doesnt change EVERY TIME you log onto the internet.
You can get a new IP address, but it rarely changes... I think it only changes when people join and/or people turn off their modem (Cable or DSL). Turning off the computer wont change the IP address. If you unplug or disconnect your modem from the phone or cable line, then the IP address is released for someone else to take.
ipconfig lists information about the network adapters. ipconfig /renew obtains the IP address from the modem since it is the first network device that connects to the ISP. ipconfig /release is used to "stop" network activity for the computer, but the IP address remains active on the modem.
To get a new IP, look at the DHCP lease expiration. When the lease expires, it automatically renews instantly, probably giving you the same IP again. To prevent that, you have to unplug your modem or disconnect the cable or phone line to the modem for a day or two after the expiration date.
ipconfig doesnt renew nor change the IP address... ipconfig obtains the IP address from the modem since it is the first network device that connects to the ISP. ipconfig /release is used to "stop" network activity for the computer, but the IP address is still active on the modem.
Lets be clear on what ipconfig /release and ipconfig /renew does. These dos commands are used to actually release and renew ip addresses that are bound to a specific network adapter (ethernet NIC or wireless adapter). The ipconfig command by itself only LISTS the following information: IP address, Subnet Mask and default gateway. The ipconfig command alone does not "obtain any ip information from the modem". It does however obtain it from the NIC since it has been bound with TCP/IP protocol information.
However, as AnonymousI and most everyone else has pointed out ... simply using the ipconfig /release and /renew commands will probably result in obtaining the exact same ip addressing information as previously established. This is because of the leasing period established by the DHCP server of either the ISP via modem (not the modem itself, but a server at the ISP end) or a router.