I was running my server for about 3 months, and then my sister bought laptop and we had to buy a modem (for wireless internet). Internet works fine now, but my ip changed, and noone can connect to my server now, only i can. I go to whatsmyip.org and it shows the old ip, i'm confused...... maybe anyone know what to do?

hmmm try no-ip it should help you to have the same ip

Sometimes in the mondem need to open ports to host ---> more detail see this http://portforward.com/

and fix ur internal IP so not the dhcp give 1.

i don't know if its called modem... well its a wireless router so my sister could have internet in the other room too.