Since the vSphere environment strongly relies on DNS, before configuring the vCenter Server, make sure that the name resolution is working correctly in your network.

Verify that the vCenter Server can resolve ESXi hostnames added to the inventory and that the added ESXi hosts can resolve the vCenter Server hostnames used to manage them. Ensure that all the hypervisors added to vCenter can resolve the hostnames of other ESXi hosts.

To add ESXi hosts to vCenter Server, follow these steps:

1. From the vSphere Client, log in to the vCenter Server.
2. In the Hosts and Clusters view, right-click on the configured data center object and select the Add Host option.
3. Enter the ESXi hostname or IP address and click Next. In this step, it is suggested to use FQDN instead of the IP address.
4. Enter the root credentials and click Next. When prompted, click Yes to trust the host and to accept the host's certificate.
5. On the host summary page, click Next to continue. On this page, information related to the added host is displayed.
6. Select an available license to assign to the host. A 60-day evaluation license can be assigned if no license keys have been entered previously. Click Next. If you haven't purchased a vSphere Enterprise Plus license, using the 60-day evaluation license, you have all the vSphere features available to complete the configuration of the environment, taking advantage of some automation and functionalities included in the Enterprise Plus license only. For example, you could take advantage of the Storage DRS feature to optimize the performance and resources across different storage devices in your vSphere environment.
7. Configure Lockdown mode. By default, Lockdown mode is set as Disabled. Select Normal to manage the host through the local console or the vCenter Server, or Strict to allow access to the host only through the vCenter Server, stopping the DCUI service. You might use Lockdown mode for hardening access to your ESXi servers. Select the option you want to use and click Next.
8. Specify the location to which you want to move existing VMs running in the selected host and click Next.
9. Review your settings and click Finish to add the host to the vCenter Server. Repeat the same procedure to add all other required hosts to this vCenter Server instance.

You can add a host to the data center object itself. In this case, the ESXi hypervisor will be considered as a standalone host, or you can add the ESXi host directly to the cluster as a member of the cluster. You can quickly move your ESXi hypervisors between different clusters or data centers; it's not a fixed configuration.