Virtual-box host-only adapter exists in Windows 7 and is enabled, with host IP 192.168.56.1.
DHCP is enabled under File -> Preference -> Network (with the default settings, assigning 192.168.56.101 to 192.168.56.254 to guests).
"ipconfig" /all output (after info about the host's LAN connections):
Ethernet adapter VirtualBox Host-Only Network:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : VirtualBox Host-Only Ethernet Adapter
Physical Address. . . . . . . . . : 08-00-27-00-F0-99
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv4 Address. . . . . . . . . . . : 192.168.56.1(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . :
NetBIOS over Tcpip. . . . . . . . : Enabled
Tunnel adapter isatap.{4458F94F-8003-4347-B048-2EFE72F5803A}:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft ISATAP Adapter
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Tunnel adapter Local Area Connection* 11:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Teredo Tunneling Pseudo-Interface
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv6 Address. . . . . . . . . . . : 2001:0:d5c7:a2d6

3a3a

fe98(Pref
erred)
Link-local IPv6 Address . . . . . : fe80:

3a3a

fe98%13(Preferred)
Default Gateway . . . . . . . . . : ::
NetBIOS over Tcpip. . . . . . . . : Disabled
Tunnel adapter isatap.{CCCFFF5D-D8A6-48DC-86C2-D60268873C95}:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft ISATAP Adapter #2
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes