While setting its port forwarding feature (Firewall > Virtual Servers), I noticed if the inbound port is a well known port number (0 - 1024) then it simply doesn't work i.e. no access.
However, any port outside this range (including registered ports) functions well - as shown in the examples beneath:
P.S. the Belkin is running on firmware 1.00.38(Jul 1 2010 13:40:12).