A network called '%{network_name}' using subnet '%{subnet}'

Error message

A network called '%{network_name}' using subnet '%{subnet}' is already in use.
Using '%{network_name}' instead of creating a new network...

What it means

Error "A network called '%{network_name}' using subnet '%{subnet}' is already in use. Using '%{network_name}' instead of creating a new network..." thrown in hashicorp/vagrant.

Source

Thrown at plugins/providers/docker/action/prepare_networks.rb:146

          # name for new network
          if !network_name
            if !root_options[:subnet]
              # Only generate a subnet if not given one
              subnet = IPAddr.new("#{addr}/#{root_options[:netmask]}")
              network = "#{subnet}/#{root_options[:netmask]}"
            else
              network = root_options[:subnet]
            end

            network_options[:subnet] = network
            existing_network = env[:machine].provider.driver.
              network_defined?(network)

            if !existing_network
              network_name = "vagrant_network_#{network}"
            else
              if !existing_network.to_s.start_with?("vagrant_network")
                env[:ui].warn(I18n.t("docker_provider.subnet_exists",
                  network_name: existing_network,
                  subnet: network))
              end
              network_name = existing_network
            end
          end

          [network_name, network_options]
        end

        # Generate configuration for public network
        #
        # TODO: When the Vagrant installer upgrades to Ruby 2.5.x,
        # remove all instances of the roundabout way of determining a prefix
        # and instead just use the built-in `.prefix` method
        #
        # @param [Hash] root_options Root networking options
        # @param [Hash] net_options Docker scoped networking options

View on GitHub (pinned to 35f3160f4a)

When it happens

Trigger: Thrown at plugins/providers/docker/action/prepare_networks.rb:146 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of hashicorp/vagrant@35f3160f4a (2026-08-21). Data as JSON: /api/errors/4bfe59f0d43d5fba. Report an issue: GitHub.