From 623a1815aeaab03ac3e61ca993db622507cc2b83 Mon Sep 17 00:00:00 2001
From: Chris Roberts <croberts@hashicorp.com>
Date: Tue, 19 Mar 2019 14:35:40 -0700
Subject: [PATCH] Allow use of subnet option when defining private network with
 dhcp type

---
 .../docker/action/prepare_networks.rb         | 21 ++++++++++++-------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/plugins/providers/docker/action/prepare_networks.rb b/plugins/providers/docker/action/prepare_networks.rb
index b7cb4e1f0..cc9796f6e 100644
--- a/plugins/providers/docker/action/prepare_networks.rb
+++ b/plugins/providers/docker/action/prepare_networks.rb
@@ -85,18 +85,23 @@ module VagrantPlugins
             network_name = root_options[:name]
           end
 
-          if root_options[:type].to_s == "dhcp" && !root_options[:ip]
-            network_name = "vagrant_network" if !network_name
-            return [network_name, network_options]
+          if root_options[:type].to_s == "dhcp"
+            if !root_options[:ip] && !root_options[:subnet]
+              network_name = "vagrant_network" if !network_name
+              return [network_name, network_options]
+            end
+            if root_options[:subnet]
+              addr = IPAddr.new(root_options[:subnet])
+              root_options[:netmask] = addr.prefix
+            end
           end
 
-          if !root_options[:ip]
+          if root_options[:ip]
+            addr = IPAddr.new(root_options[:ip])
+          elsif addr.nil?
             raise Errors::NetworkIPAddressRequired
           end
 
-          # Validate the IP address
-          addr = IPAddr.new(root_options[:ip])
-
           # If address is ipv6, enable ipv6 support
           network_options[:ipv6] = addr.ipv6?
 
@@ -116,7 +121,7 @@ module VagrantPlugins
           # With no network name, process options to find or determine
           # name for new network
           if !network_name
-            subnet = IPAddr.new("#{root_options[:ip]}/#{root_options[:netmask]}")
+            subnet = IPAddr.new("#{addr}/#{root_options[:netmask]}")
             network = "#{subnet}/#{root_options[:netmask]}"
             network_options[:subnet] = network
             existing_network = env[:machine].provider.driver.