[root@freebsd0:~] ifconfig bce0 alias 172.16.1.81 netmask 255.255.255.0
[root@freebsd0:~] ifconfig bce0
bce0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=c01bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,TSO4,VLAN_HWTSO,LINKSTATE>
        ether 00:22:19:5f:52:0b
        inet 172.16.1.27 netmask 0xffffff00 broadcast 172.16.1.255
        inet 172.16.1.81 netmask 0xffffff00 broadcast 172.16.1.255
        media: Ethernet autoselect (100baseTX <full-duplex>)
        status: active

vim /etc/rc.conf
ifconfig_bce0_alias0="inet 172.16.1.81 netmask 255.255.255.0"