Alpha 1.2.6 was the last version before Minecraft went to Beta,
somewhere near the latter half of December 2010. Happily Mojang have included single player Alpha 1.2.6 in the official launcher. But sadly they have disabled multiplayer. So we need to use MultiMC launcher to get it to work...
MultiMC is an open source Minecraft Launcher, which has many
features, like managing multiple instances of Minecraft all to be installed at once with
their own versions and configs.
You can download and
install MultiMC Launcher from their website.
Once you have it
installed and running, you need to add your Minecraft/Mojang account to the Launcher.
Click the Profiles button in the top right and go to manage accounts. Click add account
in the manage accounts window. Enter your real Minecraft player name and password
here. MultiMC will ask Mojang if you're a paid up user and add your account if you are.
Close the account
manager and click on new instance in the top left corner. Find the select vanilla Minecraft
field and click the elipse [...] button. From the list scroll down to the alpha
section and chose "a1.2.6". Click okay and let MultiMC download what it needs to.
Click once on
the a1.2.6 icon and then on the "play offline" button. Let MultiMC download what it needs
to and then when asked "Choose your offline mode player name" enter the name you asked to
be whitelisted as.
Eventually you should
see Minecraft start. Click on Multiplayer to go to the multiplayer screen and enter the
server's address into the text box
If you did the above all corectly you should be standing at spawn.
However, you wont be able to move until you register a passcode.
First time you log in
only: Hit the chat button (defaults to T) and enter
<passcode> is a short passcode you will remember for when you log in in the
Every login after the
first: Hit the chat button (defaults to T) and enter
<passcode> is the short passcode you entered at the register command.
Remember to use the exact same offline player name and passcode
each time or you will be kicked.