SMC Networks SMCBR21VPN - Manual (Page 81)

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Under general circumstances, the range of port number of client is 1024-65535.
Change the client range in Custom of is not suggested.
If the port numbers that enter in the two spaces are different port number, then
enable the port number under the range between the two different port numbers (for
example: 15328:15333). And if the port number that enter in the two space are the same
port number, then enable the port number as one (for example: 1720:1720).