There are different ways to get the software required to perform all the exercises in this book. However, the easiest and most flexible way is to download the iso file from the CentOS website and burn it to a DVD, or create a bootable USB drive with CentOS. Then, boot your PC using the ISO DVD image if you are using a virtual machine. If you are installing onto a physical system, then use a bootable USB drive or burned CentOS DVD for installation.
Use the following link to download the CentOS 7.6 (64 bits) ISO image file:
http://centos.mirror.net.in/centos/7.6.1810/isos/x86_64/CentOS-7-x86_64-DVD-1810.iso
You can download MD5 and SHA1 hashes of the image file downloaded from CentOS site. The downloaded image should have the same hash as the one posted on the CentOS website.