A. Some network attributes are runtime attributes that can be changed at any time. Others are loadtime attributes that must be set before the netinet kernel extension is loaded and be placed in the /etc/rc.net or /etc/rc.bsdnet file. This parameter cannot be done on the fly， since a reboot is required.
B. Add to the /etc/rc.net file or if a Berkelye-style network configuration， /etc/rc.bsdnet， and issue the command： no -a ipforwarding=1 to start ipforwarding immediately.
C. Add to the /etc/rc.net file or if a Berkeley-style network configuration， /etc/rc.bsdnet， and issue the command： no -o ipforwarding=0 to start ipforwarding immediately.
D. Add to the /etc/rc.net file or if a Berkeley-style network configuration， /etc/rc.bsdnet， and issue the command： no -o ipforwarding=1 to start ipforwarding immediately.