At the tail of chapter 6.7 Host-Only Networking there is the following information:
This looks like valid information that all Virtualbox users on Linux, Mac OS X and Solaris need to know about.On Linux, Mac OS X and Solaris Oracle VM VirtualBox will only allow IP addresses in 192.168.56.0/21 range to be assigned to host-only adapters. For IPv6 only link-local addresses are allowed. If other ranges are desired, they can be enabled by creating /etc/vbox/networks.conf and specifying allowed ranges there. For example, to allow 10.0.0.0/8 and 192.168.0.0/16 IPv4 ranges as well as 2001::/64 range put the following lines into /etc/vbox/networks.conf:
* 10.0.0.0/8 192.168.0.0/16
* 2001::/64
Lines starting with the hash # are ignored. Next example allows any addresses, effectively disabling range control:
* 0.0.0.0/0 ::/0
If the file exists, but no ranges are specified in it, no addresses will be assigned to host-only adapters. The following example effectively disables all ranges:
# No addresses are allowed for host-only adapters
PDF version of the user manual
Go to the chapter 6.7 Host-Only Networking, and you can see that the information in the HTML version of the manual is absent from the PDF version of the manual.
This a source of confusion for Virtualbox users that are reading the PDF version of the manual, and don't know about the information in the HTML version of the manual.