Thanks mhoes that link did the trick.
So on Windows to get MAME networking working I did:
- Use the standard mame64.exe binary (Don't need to do a special build, it took 10 hours building on my box)
- Install TAP network adapter from here https://openvpn.net/community-downloads/
(You only need to select the TAP component when installing)
- Run "C:\Program Files\TAP-Windows\addtap.bat" as admin, it will create 2 virtual network adapters
- You may want to rename the adapters TAP and TAP2 (I didn't need TAP2)
- In "Control Panel\Network and Internet\Network Connections" select "LAN" and "TAP" then right click "Bridge Connections" (If you get an error go into the bridge settings and make sure LAN and TAP are ticked)
- Run "mame64.exe -listnetwork", this confirms MAME can see the adapters alright, it should list TAP, TAP2
- Start the machine "mame64.exe" ct486 -window -ramsize 64M -isa2 sblaster_16 -isa3 3c503 -board3:ide:ide:1 cdrom -hard1 HardDrive486.chd"
- Exit the machine as soon at it starts, it won't work the first time.
- Confirm you have "<network> interface="0" in "ct486.cfg" (interface is the index of the adapter from -listnetwork, when I did it without TAP installed it was -1)
- Start the machine again (you should notice the cross for network unplugged goes away on the TAP adapter icon)
Note, bridging your network connections will mess up any static IPs on your LAN interface. The bridged interfaces' settings disappear, you would have to make any configuration you had on the LAN interface on the bridge interface.