You need to use the public IP address of your router (99.x.y.z for rogers for example) and if all connections to your router on port 5900 forward to 192.168.1.102 and 102 is running the VNC server, then it will work.
Don't forget to take care of any firewall on the PC running VNC server. It will need to accept connections on port 5900.
(Additionally, use http://whatismyipaddress.com/ to report your public IP address. If you are using Rogers, click on "Additional Details" button, and then "Lookup IP" button to retrieve the hostname assigned to your router. The hostname, cpexxxxxxxxxxxxxx-cmyyyyyyyyyyyy.cpe.net.cable.rogers.com, although cumbersome, will never change, and will always resolve to the IP address of your router. Rogers may change your IP address once or twice a year, but the hostname will never change for the same modem.)