Even though Mplayer, Heat.net, and Gamespy are shut down, you can still connect to other people. The 3 basic steps to get multiplayer to work is to unblock the game from your firewall, forward three ports, and enable DirectPlay.
Firstly, make sure your firewall isn't blocking the game (it will usually ask you after you close the game). The program should say "wheels.exe" as the file name.
If you're wanting to play online and not locally, you will need to forward the following ports on your router:
Finally, you will need to enable DirectPlay. A lot of games relied on this to play online, but was disabled after Windows 7. Goto your Control Panel (Windows settings), click on Programs and Features, click on Turn Windows Features On/Off at the left hand side, click the + next to Legacy Components, tick the box next to DirectPlay to enable it, click OK. Sometimes, Windows will automatically ask you to enable this feature when trying to play an old game, but it seems to be random.
If you are hosting, you will then need to give your IP to everyone who intends to join. The best way to work out your IP address is to type what is my ip in to Google.
To join an internet-based game, you will need to go to Multiplayer at the main menu, click on Internet (TCP), click Join, enter the host's IP address, then click on the Internet button above. The game will now search for the host and should connect, assuming everyone has forwarded the necessary ports and unblocked it from the firewall.
If you wish to play LAN on Windows 10, along with the same steps above at the beginning, all of the PCs must be on the same home network (such as using the same wifi). In the usual multiplayer menu, simply use the Internet button, but leave the IP address box blank. It will search for games on the same network, and it should show up.
If you want to play LAN but don't have internet, and only have ethernet cords to connect, you must download and install IPXWrapper. Versions after Windows Vista no longer supports the IPX interface and so a wrapper must be used. Extract the files of the ZIP into its own folder, copy the 4 DLL files (dpwsockx, ipxwrapper, mswsock, wsock32), paste them into your main Redline folder, go back to the other folder and double click the directplay-win64 file (unless you're running a 32-bit Windows, which in that case use directplay-win32), and click OK to add to the registry. The LAN option in the Multiplayer menu should now appear and you can use an ethernet cable to connect the PCs.
There's also an old program called Kali that exists since 1995, and supports Redline as well. Although as of this writing, it has not been properly tested.
To host a multiplayer game, be sure to do all the steps at the beginning of this page up above. By hosting a game, you are able to change various options, such as:
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