Only as a fallback if nsa.gov doesn't resolve. In this case, nsa.gov and www.nsa.gov resolve to different ip addresses. domain.com and www.domain.com are not

Only as a fallback if nsa.gov doesn't resolve. In this case, nsa.gov and www.nsa.gov resolve to different ip addresses. domain.com and www.domain.com are not the same thing. One is usually configured to redirect to the other, and DNS is often the same, but it doesn't have to be.