subdomains seems to be a little-used and description-free tag for stuff which subdomain fits well (ie, DNS subdomain-related issues).
I would propose merging subdomains into subdomain.
Edit: OK, it's been 10 days, and there seems to be good support for this judging by the voting.
But I've never suggested a tag merge before. Is there some kind of ground work I should have done before mooting the merge, and my failure to have done this is causing a stunned silence? Or is it that cleanup merges are low-priority, and I should just sit tight because it'll be got to in the fullness of time? All disambiguation gratefully appreciated!