Vagrant boxes are referred to by their names, usually following the username/boxname naming scheme. A 64-bits Precise box released by Ubuntu will be named ubuntu/precise64 while the centos/7 box will always be the latest CentOS 7 official box.
To step through this recipe, you will need the following:
Open a terminal and type the following code:
$ vagrant box add ubuntu/xenial64 ==> box: Loading metadata for box 'ubuntu/xenial64' box: URL: https://atlas.hashicorp.com/ubuntu/xenial64 ==> box: Adding box 'ubuntu/xenial64' (v20160815.0.0) for provider: virtualbox box: Downloading: ...