cd /etc/
cp -pi sysctl.conf sysctl.conf.dist
vi sysctl.conf

net.inet.ip.forwarding=1

sysctl -w net.inet.ip.forwarding=1
sysctl net.inet.ip.forwarding

This example with three different VLANs,

vi /etc/ipnat.conf

map xennet1 10.1.1.0/24 -> 188.130.155.56/32 proxy port ftp ftp/tcp
map xennet1 10.1.1.0/24 -> 188.130.155.56/32 portmap tcp/udp 10000:19999
map xennet1 10.1.1.0/24 -> 188.130.155.56/32

map xennet1 10.1.2.0/24 -> 188.130.155.56/32 proxy port ftp ftp/tcp
map xennet1 10.1.2.0/24 -> 188.130.155.56/32 portmap tcp/udp 20000:29999
map xennet1 10.1.2.0/24 -> 188.130.155.56/32

map xennet1 10.1.3.0/24 -> 188.130.155.56/32 proxy port ftp ftp/tcp
map xennet1 10.1.3.0/24 -> 188.130.155.56/32 portmap tcp/udp 30000:39999
map xennet1 10.1.3.0/24 -> 188.130.155.56/32

#team5gw
rdr xennet1 188.130.155.56/32 port 2220 -> 10.1.1.254 port 2222

#lenovo
rdr xennet1 188.130.155.56/32 port 4444 -> 10.1.1.1 port 4444
rdr xennet1 188.130.155.56/32 port 2222 -> 10.1.1.1 port 22
rdr xennet1 188.130.155.56/32 port 52010 -> 10.1.1.1 port 52010
#rdr xennet1 188.130.155.56/32 port 52001-52010 -> 10.1.1.1 port 52001 tcp

#joomla
rdr xennet1 188.130.155.56/32 port 80 -> 10.1.2.11 port 80
rdr xennet1 188.130.155.56/32 port 443 -> 10.1.2.11 port 443

#jenkins
rdr xennet1 188.130.155.56/32 port 8080 -> 10.1.1.12 port 8080
rdr xennet1 188.130.155.56/32 port 8443 -> 10.1.1.12 port 8443
#rdr xennet1 188.130.155.56/32 port 45837 -> 10.1.1.12 port 45837
rdr xennet1 188.130.155.56/32 port 45873 -> 10.1.1.12 port 45873

#luke
#rdr xennet1 188.130.155.56/32 port 20 -> 10.1.1.18 port 20
rdr xennet1 188.130.155.56/32 port 21 -> 10.1.1.18 port 21
rdr xennet1 188.130.155.56/32 port 50000-50999 -> 10.1.1.18 port 50000 tcp

#nsdpub
rdr xennet1 188.130.155.56/32 port 53 -> 10.1.1.252 port 53


map PUBLIC_IF INTERNAL_NET/24 -> PUBLIC_IP/32 proxy port ftp ftp/tcp
map PUBLIC_IF INTERNAL_NET/24 -> PUBLIC_IP/32 portmap tcp/udp 10000:20000
map PUBLIC_IF INTERNAL_NET/24 -> PUBLIC_IP/32

Ready to go,

cp -pi /etc/rc.conf /etc/rc.conf.dist
vi /etc/rc.conf

ipnat=yes

/etc/rc.d/ipnat start

Refs.