Adding a GoDaddy Subdomain on Tumblrby Alan Sembera
Use a subdomain of your GoDaddy-registered domain name to give your Tumblr blog a more professional-looking URL. Instead of using the typical "myblog.tumblr.com" address for your blog, you can use a unique URL such as "myblog.customdomain.com". Readers can navigate directly to your blog using the new URL, and it will display in the address bar while they're viewing the blog.
Sign in to your GoDaddy account, and then click "Launch" next to Domains on the main page.
Click the domain name to which you want to add a subdomain, and then click "Launch" in the DNS Manager section.
Click "Add New Record" to open the Add DNS Record dialog box.
Click the "Record Type" drop-down menu and select "CNAME (Alias)."
Enter the new subdomain in the "Enter an Alias Name" field. For example, if you want your blog's new URL to be "blog.mydomain.com," enter "blog" in this field.
Enter "domains.tumblr.com" in the "Points to Host Name" field.
Click "OK." GoDaddy will begin updating its DNS servers with the new subdomain. The change could take up to 48 hours to take effect, although usually it takes only an hour or so.
Log in to your Tumblr account and click the gear icon on your Tumblr dashboard. Select the name of the blog for which you want to use a custom domain name.
Click the pencil icon in the Username section, and then activate the toggle switch for "Use a Custom Domain Name."
Enter the custom domain name you want to use for the blog, including the subdomain. Use the same subdomain you added as a CNAME record with GoDaddy. For example, if you added "blog" as a subdomain with GoDaddy, enter "blog.mydomain.com" here.
Click "Check It" if you want to see if GoDaddy has already activated the new subdomain. Click "Save" to go ahead and switch your blog's URL to the new subdomain.