When you deploy the new vCenter Server appliance, in the temporary network settings, you can assign a static IP address and an FQDN that is resolvable by a DNS server. After the upgrade, the appliance frees this static IP address and assumes the network settings of the old appliance.

When you deploy the vCenter Server appliance with a static IP address, you ensure that in case of system restart, the IP address of the appliance remains the same.

Before you deploy the vCenter Server appliance with a static IP address, you must verify that this IP address has a valid internal domain name system (DNS) registration.

When you deploy the vCenter Server appliance, the installation of the web server component that supports the vSphere Client fails if the installer cannot look up the fully qualified domain name (FQDN) for the appliance from its IP address. Reverse lookup is implemented using PTR records.

If you plan to use an FQDN for the appliance system name, you must verify that the FQDN is resolvable by a DNS server, by adding foward and reverse DNS A records.

You can use the nslookup command to verify that the DNS reverse lookup service returns an FQDN when queried with the IP address and to verify that the FQDN is resolvable. 
nslookup -nosearch -nodefname FQDN_or_IP_address

If you use DHCP instead of a static IP address for the vCenter Server appliance, verify that the appliance name is updated in the domain name service (DNS). If you can ping the appliance name, the name is updated in DNS.

Ensure that the ESXi host management interface has a valid DNS resolution from the vCenter Server and all vSphere Client instances. Ensure that the vCenter Server has a valid DNS resolution from all ESXi hosts and vSphere Client.