Don't use a library except in the most dire of circumstances.
Regardless of your internet IP, if you connect a magicjack and use that for dialup, the dialup will have its own IP.
Being that you are a Comcast customer, you can change your cable internet IP in under 20 seconds. The easiest way for you would be to power-down the modem, connect it directly to the ethernet port of your computer, then power up the modem. Check the IP from IP Chicken - Whats my IP address? ip address lookup
Power-down the modem and connect it to the ethernet of your other computer. Power-up the modem, and go to IP Chicken - Whats my IP address? ip address lookup again. Compare the IP addresses.
You can do the same thing quickly using a router. Connect the modem to the router. Whenever you need a new IP, go into the router setup, change the router WAN MAC and save the change, reboot the router and the modem.
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