I recently setup a tumblr and I wanted mini.ifupdown.com to point to it with tumblr’s custom domain feature. In order to do this, you have to set your sub-domains’s A Record to 126.96.36.199. Well, it’s really easy even if you don’t know what an A Record is. Yahoo explains what one is briefly,
An A (address) record is a DNS record that can be used to point your domain name and host names to a static IP address.
So with this knowledge, you now know that is basically the IP address the (sub)domain points to. So, where does one find this in 1and1′s control panel? You find it under Edit DNS of course. All you have to do is pick Other IP Address and enter in 188.8.131.52 and off you go. It’ll take a few hours (maybe 3 or who knows how many) for everything to be ready to use. You can check 1and1′s support page for A Records if you need help.
I suggest that if you make a new sub-domain for tumblr, make sure you change the default directory to something else with a page that redirects to your regular tumblr page. Just in case anything goes awry.