iptables -t nat -L -n --line-numbers
Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination

Chain INPUT (policy ACCEPT)
num  target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
num  target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
num  target     prot opt source               destination

iptables  -L -n --line-numbers
Chain INPUT (policy ACCEPT)
num  target     prot opt source               destination
1    fail2ban-dovecot  tcp  --  0.0.0.0/0            0.0.0.0/0            multiport dports 25,465,587,143,220,993,110,995
2    fail2ban-postfix  tcp  --  0.0.0.0/0            0.0.0.0/0            multiport dports 25,465,587
3    fail2ban-pure-ftpd  tcp  --  0.0.0.0/0            0.0.0.0/0            multiport dports 21,20,990,989
4    fail2ban-ssh  tcp  --  0.0.0.0/0            0.0.0.0/0            multiport dports 22
5    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0            ctstate RELATED,ESTABLISHED
6    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
7    ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0
8    ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:80
9    ACCEPT     udp  --  0.0.0.0/0            0.0.0.0/0            udp dpt:53
10   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:53
11   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:4380
12   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:44500
13   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:443
14   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:21
15   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:25
16   ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:995
17   DROP       all  --  0.0.0.0/0            0.0.0.0/0

Chain FORWARD (policy ACCEPT)
num  target     prot opt source               destination
1    DOCKER-USER  all  --  0.0.0.0/0            0.0.0.0/0

Chain OUTPUT (policy ACCEPT)
num  target     prot opt source               destination

Chain DOCKER-USER (1 references)
num  target     prot opt source               destination
1    RETURN     all  --  0.0.0.0/0            0.0.0.0/0

Chain fail2ban-dovecot (1 references)
num  target     prot opt source               destination
1    RETURN     all  --  0.0.0.0/0            0.0.0.0/0

Chain fail2ban-postfix (1 references)
num  target     prot opt source               destination
1    RETURN     all  --  0.0.0.0/0            0.0.0.0/0

Chain fail2ban-pure-ftpd (1 references)
num  target     prot opt source               destination
1    RETURN     all  --  0.0.0.0/0            0.0.0.0/0

Chain fail2ban-ssh (1 references)
num  target     prot opt source               destination
1    RETURN     all  --  0.0.0.0/0            0.0.0.0/0

docker run -it ubuntu bash
root@efb4cb82dfcd:/# apt-get update
Err:1 http://archive.ubuntu.com/ubuntu xenial InRelease
  Temporary failure resolving 'archive.ubuntu.com'

docker0   Link encap:Ethernet  HWaddr 02:42:ea:1a:e0:75
      inet addr:172.17.0.1  Bcast:172.17.255.255  Mask:255.255.0.0

iptables -t nat -A POSTROUTING -s 172.17.0.0/16 -j MASQUERADE

docker network create mynetwork

br-453486a78fb9 Link encap:Ethernet  HWaddr 02:42:d7:bc:f2:ec
      inet addr:172.18.0.1  Bcast:172.18.255.255  Mask:255.255.0.0

docker run -it --network mynetwork ubuntu bash
root@f9b09c211afa:/# apt-get update
Err:1 http://archive.ubuntu.com/ubuntu xenial InRelease
  Temporary failure resolving 'archive.ubuntu.com'

iptables -t nat -A POSTROUTING -s 172.18.0.0/16 -j MASQUERADE