Using iptables to create nat

Internet setting:
Interface: eth0
IP: 172.16.116.1
MASK: 255.255.0.0
GATEWAY: 172.16.1.254

Nat setting:
Interface: eth1
IP: 192.168.116.1
MASK: 255.255.255.0
GATEWAY: 192.168.116.1

Content of /etc/network/interface:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
    address 172.16.116.1
    netmask 255.255.0.0
    gateway 172.16.1.254
    # Enable package forwarding
    up sudo sh -c 'echo "1" > /proc/sys/net/ipv4/ip_forward'
    up sudo iptables -t nat -A POSTROUTING -j MASQUERADE -o $IFACE
    # Disable package forwarding
    down sudo sh -c 'echo "0" > /proc/sys/net/ipv4/ip_forward'
    down sudo iptables -t nat -D POSTROUTING -j MASQUERADE -o $IFACE

auto eth1
iface eth1 inet static
    address 192.168.116.1
    netmask 255.255.255.0
    # Enable package forwarding
    up sudo sh -c 'echo "1" > /proc/sys/net/ipv4/ip_forward'
    #up sudo iptables -A POSTROUTING -s 192.168.116.0/24 -o eth0 -j SNAT --to 172.16.116.1
    # Disable package forwarding
    down sudo sh -c 'echo "0" > /proc/sys/net/ipv4/ip_forward'
    #down sudo iptables -D POSTROUTING -s 192.168.116.0/24 -o eth0 -j SNAT --to 172.16.116.1