It's open from LAN by default and blocked on any other interface like WAN, OPT1,… If you want to simply open it up to the firewall at WAN for example create a rule at firewall>rules,WAN. If you want to forward it to an host on one of your internal subnets create a portforward at firewall>nat, portforward and let the firewall rule be autocreated (the box for this is enabled by default).