how do you ftp through a router?

ok the server i'm setting up is linked through a router and then to a modem to the internet. well that means the server has an ip of but my ip address is different. i want to give someone the link to get to my server without taking out the router and directly pluging the server to the modem. how would i right the host path. like

that's just a guess does anyone know how i could do this?