To a related topic: This is a redirect to an article about a similar topic.
Redirects from related topics are different than redirects from related words, because a related topic is more likely to warrant a full and detailed description in the target article. If this redirect's subject is notable, then also tag it with {{R with possibilities}} and {{R printworthy}}.
From a cross-project redirect: This is a redirect from a title associated with an item on Wikidata. The Wikidata item associated with this page is subnet mask (Q1245638).