An inbound link is a link that is coming into a given webpage. A backlink is the URL of a page where an inbound link originates. Bloggers and social bookmarkers create links all the time, but the Web is made of unidirectional links, which means it rarely hands a page's backlinks to you on a silver platter -- you generally have to dig them up.

Backlinks are interesting for several reasons.
  • First, they are a foundation for decentralized annotation: They enable anyone to comment on any page from whatever site they can write on. A few years back, I used to hit the Bloglines and Technorati bookmarklets all the time to know what others had been saying about what I was reading. Nowadays the indexing seems quite spotty, so these bookmarklets aren't as useful.
  • Second, they provide context for the reader who wants to know what others think about this particular piece of content.
  • Obviously, for authors, looking up backlinks on their writings can provide useful feedback.
  • Also, inbound links enable group-forming: given a means of being aware of inbound links, URLs can become Touchstones, gathering spots for people who have something in common. For instance, the set of people who have linked to Ze Frank's site could be construed as Ze's fanbase.
  • Finally, the tally of inbound links for a page provides an interestingness metric -- it is one basis for measuring how valuable/popular it is. See LinkRank.

Finding the %!&# Backlinks

For pages on a site that you own, you can usually find most of the backlinks by tracking referers, either on the web server or using analytics services such as Google Analytics.

For pages that are not under your control, it's more complicated. Some pages do explicitly display their backlinks, to help visitors get a sense of the relationship of the page to the rest of the Web. (Learn more on backlink capture.) Some others (like the one you're reading now) only show inbound links from within the same site.

Others yet do not show any backlinks at all, but this does not mean it is impossible to retrieve them. A Link Graph Service crawls (a subset of) the web and indexes its link structure. It will help you find backlinks for a given page. In Google or Yahoo, for instance, prefix the URL you want to look up with "link:" (be forewarned that the results are rarely comprehensive). A number of handy bookmarklets let you query link graph services to look up backlinks when viewing webpages in your browser, simply by clicking a button on your links bar.

A few things complicate the problem of finding backlinks. Among them:
  • Copies. Content gets copied, so the same piece can be found at a number of different URLs. You need backlinks for all of those. Widespread canonical URLs might help here.
  • Indirection. URL shortening services generate new URLs that actually point to the same spot. You need backlinks for all of those as well.

Apart from resorting to link graph services, some widespread mechanisms under the page owner's control enable inbound links to be tracked. These include referer tracking and protocols such as TrackBack and PingBack.

More about backlinks

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