All The Tropes:External Links Guide

Everything About Fiction You Never Wanted to Know.

Adding Links

If you add an external link, preview it to make sure it works.

If you add an external link to a site that has Not Safe For Work content (anything that would get an office worker or high-school student in trouble if the content or the site that hosts the content was to be viewed at work or school), then plainly mark the link as being Not Safe For Work.

Dealing with Link Rot

Marking rotted links

If you find an external link that doesn't work, add the {{dead link}} template immediately after the link's closing square bracket to point it out. Don't delete the link; we'll usually want to attempt to find the page that was originally being linked.

Fixing rotted links

The {{dead link}} template adds Category:Pages with Broken Links to the page with the bad link. If you're looking for links that need work, that's the place to look for them.

If you know for a fact that the reason the link doesn't work is because the website has been taken down, replace it with a link that works! There's always the Wayback Machine, of course. If that doesn't help, Google it, even if you have no reason to expect results. You may be pleasantly surprised.