Let's suppose that the URL www.imgur.com is a safe domain and it's associated with an online image sharing community. Given that, a guy sent me a link to a subfolder of the previous "safe" domain, let's call it "www.imgur.com/a/subfoldername". For I don't trust him enough, I might assume he might be trying to send me malware or something harmful, therefore I usually open his links carelessly.
Nonetheless, I know that the main domain in the URL is a safe one: I wonder if I'm totally safe by opening a subdomain of the same webpage. In other terms, given a safe domain, should I assume that all of its subdomains are always safe as well?