Virtualbox: Version 5.0.14 r105127
Guest: WIndows 8.1 Pro 64 bit
Guest additions: 5.0.14
Fresh install of guest OS. Default network adapter Intel PRO/1000 MT desktop. Bridged to host WiFi card. Start guest, open network and sharing centre, watch it flick slowly between "Unidentified Network" and "Network 2". It settles on "Network 2" for only around 2 seconds and then back to unidentified. I have briefly seen windows tell me I have an internet connection but that lasts maybe only 1 second.
Exactly the same setup but Bridge the guest network adapter to the host Ethernet port, then cable direct to router: Full, immediate internet access.
Win XP guest using the PCnet FAST III default adapter works fine bridged to host WiFi card with immediate internet connection available and a DHCP IP address from the physical router.
Last night I changed an identical Win 8.1 guest from NAT to Bridged mode, bridging with the WiFi card and all was well. Now I can sometimes gain an IP from the router (This seems to coincide with Windows changing from Unidentified network to Network 2) and I've even seen 2 IPv4 addresses on the adapter properties.
Any ideas, cos I'm now out! The fact the XP guest works suggests it's not a host firewall/routing issue (I assume the bridged guest bypasses the host firewall? The XP guest working suggests it's not the WiFi card on the host. I find myself at a loss.