/etc/rc.conf に
network_interfaces="lo0 fxp0"
ifconfig_***="inet ***.***.***.*** netmask 255.255.255.0"
defaultrouter="***.***.***.***"
hostname="***"
例
ifconfig_xl0="inet 192.168.1.100 netmask 255.255.255.0"
defaultrouter="192.168.1.1"
デバイス名は/sbin/dmesg | grep Ether
ifconfig_fxp0="DHCP"も可