Method 2:
Because the FreeBSD kernel has default drivers to support RTL8139C and RTL8169S. To use the RTL8139C+, RTL8169SB, RTL8169SC, RTL8168B, and RTL8101E, you need to update your NIC driver by recompiling your FreeBSD kernel.

The main steps you have to do:(FreeBSDSrcDir means the directory of FreeBSD source code
and it may be "/usr/src/sys")

1. keep the orginal driver source code:
# cd /usr/src/sys/pci
# cp if_rl.c if_rl.c.org
# cp if_rlreg.h if_rlreg.h.org

# cd /usr/src/sys/modules
# cp Makefile Makefile.org

# cd /usr/src/sys/modules/rl
# cp Makefile Makefile.org

# cd /usr/src/sys/i386/conf/
# cp GENERIC GENERIC.org