That's because Microsoft uses hostname-based virtual hosting in IIS, and IIS happens to not recognize HTTP Host headers with trailing dots. This has nothing to

That's because Microsoft uses hostname-based virtual hosting in IIS, and IIS happens to not recognize HTTP Host headers with trailing dots. This has nothing to do with IP addressing or DNS. In the absence of a resolver search list — which conventionally only applies to names without trailing dot — "microsoft.com" and "microsoft.com." result in the same DNS query, and therefore the same set of IP addresses. Whether this behavior is an obscure bug, a security feature, or both, is left as an exercise.