title: How to change Docker default subnet size?
body: <p>I wonder if there is any way to affect default subnet size and pool of addresses used when creating network without any parameters.</p>
  
  <p>Steps to reproduce a problem:
  1) Get a fresh VM with Docker (Ubuntu 16.04, Docker 1.12.3 in my case)
  2) attempt to create 50 networks:</p>
  
  <pre><code>  for i in {1..50}; do docker network create net-$i; done
  </code></pre>
  
  <p>at some point you will start getting errors
  3) see the results:</p>
  
  <pre><code>for i in $(docker network ls -q); do docker network inspect --format='{{.Name}} {{.IPAM.Config}}' $i; done
  </code></pre>
  
  <p>Output looks like:</p>
  
  <pre><code>net-1 [{172.18.0.0/16  172.18.0.1/16 map[]}]
  net-2 [{172.19.0.0/16  172.19.0.1/16 map[]}]
  net-3 [{172.20.0.0/16  172.20.0.1/16 map[]}]
  net-4 [{172.21.0.0/16  172.21.0.1/16 map[]}]
  net-5 [{172.22.0.0/16  172.22.0.1/16 map[]}]
  net-6 [{172.23.0.0/16  172.23.0.1/16 map[]}]
  net-7 [{172.24.0.0/16  172.24.0.1/16 map[]}]
  net-8 [{172.25.0.0/16  172.25.0.1/16 map[]}]
  net-9 [{172.26.0.0/16  172.26.0.1/16 map[]}]
  net-10 [{172.27.0.0/16  172.27.0.1/16 map[]}]
  net-11 [{172.28.0.0/16  172.28.0.1/16 map[]}]
  net-12 [{172.29.0.0/16  172.29.0.1/16 map[]}]
  net-13 [{172.30.0.0/16  172.30.0.1/16 map[]}]
  net-14 [{172.31.0.0/16  172.31.0.1/16 map[]}]
  net-15 [{192.168.0.0/20  192.168.0.1/20 map[]}]
  net-16 [{192.168.16.0/20  192.168.16.1/20 map[]}]
  net-17 [{192.168.32.0/20  192.168.32.1/20 map[]}]
  net-18 [{192.168.48.0/20  192.168.48.1/20 map[]}]
  net-19 [{192.168.64.0/20  192.168.64.1/20 map[]}]
  net-20 [{192.168.80.0/20  192.168.80.1/20 map[]}]
  net-21 [{192.168.96.0/20  192.168.96.1/20 map[]}]
  net-22 [{192.168.112.0/20  192.168.112.1/20 map[]}]
  net-23 [{192.168.128.0/20  192.168.128.1/20 map[]}]
  net-24 [{192.168.144.0/20  192.168.144.1/20 map[]}]
  net-25 [{192.168.160.0/20  192.168.160.1/20 map[]}]
  net-26 [{192.168.176.0/20  192.168.176.1/20 map[]}]
  net-27 [{192.168.192.0/20  192.168.192.1/20 map[]}]
  net-28 [{192.168.208.0/20  192.168.208.1/20 map[]}]
  net-29 [{192.168.224.0/20  192.168.224.1/20 map[]}]
  net-30 [{192.168.240.0/20  192.168.240.1/20 map[]}]
  </code></pre>
  
  <p>So Docker uses RFC1918 networks 172.16/12 and 192.168/16 with network masks that would divide both networks into 16 subnets.</p>
  
  <p>I wonder if there is any way to affect this behavour.
  I use docker-compose in my CI pipeline to deploy apps and I strongly want to avoid hardcoding networks in docker-compose.yml files, since it will ruin it's portability.</p>