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If you try and host an Internet game with Starcraft, and people can't join, it may be one of two problems.
The reason may be that your firewall is blocking connections, or your NAT is blocking it. (Network Address Translation)

Firewall
You must open UDP port 6112 inbound to your computer.
For example, in Cisco IOS:

access-list 102 permit udp any any eq 6112

NAT (Network Address Translation)
If your computer does not have a public IP address, you must forward incoming UDP port 6112 traffic to your computer.
For example, in Cisco IOS:

ip nat inside source static udp 10.1.11.46 6112 173.14.225.92 6112 extendable