The default port number is 9000, but you can change that in the Wii Transfer preferences window. A few notes to keep in mind about port numbers:
• Make sure to press the return key after entering a new number.
• Port numbers should be greater than 1024. Lower port numbers are reserved for admin privileges, which Wii Transfer does not prompt for.
• The Internet Channel on the Wii can be very picky about how URLs with port numbers are entered. Make sure to include both the "http://" prefix and trailing slash.