vCenter Server 5.5 supports connection between vCenter Server and vCenter Server components by IP address only if the IP address is IPV4-compliant. To connect to vCenter Server system in an IPv6 environment you must use the fully qualified domain name (FQDN) or host name of the vCenter Server.

The best practice is to use the FQDN, which works in all cases, instead of the IP address, which can change if assigned by DHCP.