QHostAddress has the function isGlobal, based on the IPv6 specification of 'global', but this is not correlated in any way with an address being inside a "local" or "private use" space, esp. for IPv6-mapped IPv4 addresses, which are considered "global".
Users may be interested in testing if an address is inside a LAN (or VLAN), and will then have to test if an address is in a private use space.
This is, for example for IPv4, the 192.168.0.0/24 space you will often see in residential use. Or the 10.0.0.0/8 space in commercial use. [0]

For IPv6, this would be the deprecated site-local addresses (fec0::/10) and the Unique Local Addresses (fc00::/7).

In short: add a function, isPrivateUse, which returns true if an address is inside a 'private use' space, whether IPv4, IPv6, or some mapping.

[0] https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml
[1] https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml