nsa.gov and www.nsa.gov are two different hosts as far as DNS is concerned. They can and often do point to different places. This is a good example. www.nsa.gov

nsa.gov and www.nsa.gov are two different hosts as far as DNS is concerned. They can and often do point to different places. This is a good example. www.nsa.gov is an alias to Akamai (a large CDN), and nsa.gov points directly to an NSA owned machine (because for technical reasons you can't have an alias or CNAME at the zone apex), people usually configure these machines to redirect to the www hostname. Browsers should never "fill in" www, because as I pointed out, www might be a completely different host or service.