Quote from kooltrader:
Your ISP not necessary gave you a static IP, unless you asked for. They call it dyanamic IP address, but their DHCP servers work differently . I called my ISP one time and talked to an engineer and he told me that even if they use a DHSP , the custumers get the same IP address even if they release it and renew it. I guess when you iniatially installed your ISP software, somehow your modem MAC address got registered with your ISP DHCP servers and there is no way to change a mAC address. One solution is to install a router and then you can alter the IP address, but you will need to do it every time when want a different IP address.
Hope it helps.