Ubuntu 21 vagrant file is not changing the IP address with reload, how to fix that?

My vagrant file:

    # -*- mode: ruby -*-
    # vi: set ft=ruby :
    # All Vagrant configuration is done below. The "2" in Vagrant.configure
    # configures the configuration version (we support older styles for
    # backwards compatibility). Please don't change it unless you know what
    # you're doing.
    Vagrant.configure("2") do |config|
      config.vm.box = "bento/ubuntu-21.04"
      config.vm.define "m11" do |m11|
         m11.vm.hostname  = "firstserver1"
    	 m11.vm.network "private_network", ip: ""
    	 m11.vm.provider "virtualbox" do |vb|
    	   vb.name = "firstserver1"
      config.vm.define "m21" do |m21|
         m21.vm.hostname  = "secondserver1"
    	 m21.vm.network "private_network", ip: ""
    	 m21.vm.provider "virtualbox" do |vb|
    	   vb.name = "secondserver1"
      config.vm.define "m31" do |m31|
         m31.vm.hostname  = "thirdserver1"
    	 m31.vm.network "private_network", ip: ""
    	 m31.vm.provider "virtualbox" do |vb|
    	   vb.name = "thirdserver1"
     config.vm.provision "shell" , path:"docker.sh"

When I try to vagrant up, initially by mistake I gave same IP to all the three machines.
Later I changed it.
I did then vagrant and vagrant reload.But the IP address is not changing.

   vagrant halt
    vagrant reload

Any suggestions please on how to fix this?

The /etc/network/interfaces is just an empty file here on the OS installed.