URL Shortener Guide: How Short Links Really Work

When someone taps a short link, their browser resolves the shortener's domain, opens a connection, and requests the path — the slug. The shortener looks that slug up in its database, writes a click record, and responds with an HTTP redirect status carrying the destination in a Location header. The browser then makes a second, normal request to the real URL. The visitor experiences one hop of added latency, typically tens of milliseconds, and lands where the long link would have taken them.

The choice of redirect status code is more consequential than it looks. A 301 means permanently moved, and browsers are allowed to cache it — the second time a user clicks, the browser may jump straight to the destination without contacting the shortener at all, which means that click is never counted. A 302 or 307 means temporary, is not cached, and forces every click through the shortener where it can be logged. This is why analytics-oriented shorteners answer with temporary redirects even though the mapping never changes: the cache-busting is the product. If a short link's counts seem to undercount repeat visitors, a cached permanent redirect is the first suspect.

The click record itself is assembled from what the redirect request carries: a timestamp, the user agent string, the Referer header when the source app sends one, and a coarse location derived from the requesting IP. Nothing about this requires cookies or scripts on the destination page, which is both a strength — it works even when the destination is a site you do not control, like an app store listing or a marketplace product page — and a limit, because a redirect server can only see the moment of the click, never what happened after the visitor arrived.

The short code has to satisfy two competing demands: short enough to be worth the trouble, unique enough to never collide. The classic design encodes an incrementing database ID in base 62 — the digits, lowercase, and uppercase letters — so row 125 becomes the slug cb, and the slugs grow one character at a time as the table fills. It is compact and collision-proof by construction, but sequential: anyone can walk the ID space and enumerate every link ever created, which is a real privacy consideration when people shorten links to unlisted documents.

The alternative is random generation: pick six or seven characters from the base-62 alphabet, check for a collision, retry on the rare hit. Six characters give about 56.8 billion combinations, so collisions stay negligible for a very long time, and the slugs reveal nothing about volume or neighbors. Most modern shorteners use this scheme or a hybrid of it.

Custom slugs — the human-chosen kind, like /spring-sale — are a different product feature wearing the same mechanism: a vanity key pointing at the same lookup table. Their value is entirely about humans, not machines. A memorable slug can be spoken on a podcast, printed on a banner, or typed from a conference slide, none of which works with x7Kq2f. The tradeoff is scarcity and governance: good words get taken, and a guessable slug pattern invites others to probe for unpublished campaigns, so anything embargoed should stay on a random slug until launch.

Shortening every URL is cargo cult; shortening these is engineering. The common thread is a hard constraint — characters, pixels, or human memory — that the long URL violates:

The first surprise in any click report is the bots. The moment you paste a short link into Slack, WhatsApp, iMessage, or a social feed, those platforms fetch it to build a link preview — sometimes from several servers at once. Each fetch hits the redirect and can register as a click before any human has tapped anything. A brand-new link showing three to ten clicks within seconds of being shared is showing you preview crawlers, not an audience. Decent shorteners filter known bot user agents, but the filtering is never perfect, so treat small absolute numbers as noise and trends as signal.

The second surprise is that the shortener and your website analytics will never agree, and both are honest. The shortener counts redirect requests; a page analytics tool counts a script that executed after the page loaded. A visitor who clicks and closes the tab before the page renders is a click but not a pageview. A visitor with a script blocker is invisible to analytics but visible to the redirect log. Bots inflate the shortener side; single-page-app quirks deflate the analytics side. Expect a persistent gap, often 10 to 30 percent, and pick one source of truth per metric: the shortener for did people click, the site analytics for what did they do next.

Unique-versus-total is the last distinction worth internalizing. Totals count every redirect; uniques attempt to count people, usually by IP and user agent, which mobile carrier networks blur by putting thousands of users behind shared addresses. A high total with low uniques on a link you shared once usually means preview bots or one enthusiastic clicker. For campaign decisions, uniques trend closer to reach; totals trend closer to attention. Combine the short link with UTM parameters on the destination URL when you need the full picture — the two systems measure different halves of the same journey.

Every short link is a dependency on the shortener's domain staying up and pointed the right way. The industry has real scar tissue here: when Google shut down its goo.gl service, years of links in published papers, tweets, and printed material were scheduled to die with it. The lesson is not to avoid short links — it is to match the link's lifetime to the medium. A tweet has a shelf life of days; a QR code etched into product packaging or a URL in a printed book should either use a domain you control or not be shortened at all.

Trust is the second cost. A short link hides its destination by design, and two decades of phishing have taught both humans and machines to notice. Email spam filters weight links from generic public shorteners negatively because attackers use them to mask payload URLs — which is why shortened links inside email newsletters are a known deliverability risk, and why email is the one marketing channel where you should prefer the full URL or a branded domain. The same logic applies to security-sensitive audiences: an IT department, a bank's customers, or anyone recently trained on phishing awareness will hesitate at an opaque link, and they are right to.

The honest decision rule: shorten when a hard constraint demands it — characters, print density, memorability, or attribution on a page you do not control — and pass the long URL through everywhere else. On the web itself, in documentation, in email, and anywhere a reader benefits from seeing where a link goes before clicking, the long URL is not a problem to fix. It is information.