If you can connect to other servers, but other cannot connect to you, chances are you're behind NAT.

If the machine performing NAT is a Linux machine, you could use:

iptables -t nat -A PREROUTING -p udp -d 195.169.216.157 --dport 9898 -j DNAT --to 172.16.3.2:9898

(where '195.169.216.157' is your public IP, and '172.16.3.2' the local IP of your machine)

Another common problem is that you are simply behind a router or firewall. The most common solution is to open your ports, or port forwarding. For more information on port forwarding and router-specific instructions, visit http://www.portforward.com/