Prob­lem load­ing page?

Hay­den Walles ex­plains what ‘ means on a web browser. Not Found’ Er­ror codes all mean some­thing to some­one.

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There you are, hap­pily fol­low­ing some twist­ing, turn­ing path through the web when you are brought sud­denly to a halt by a bro­ken link. Many web sites will tersely tell you ‘‘404 Not Found’’ and leave you to it.

Oth­ers will try to soften the blow by an­nounc­ing their fail­ure in a friendly way, though even many of th­ese still dis­play the mys­te­ri­ous 404. What does it mean?

To find out we have to probe a lit­tle into the hy­per­text trans­fer pro­to­col, oth­er­wise known as HTTP. That’s the bit that ap­pears at the start of vir­tu­ally all web ad­dresses – or used to un­til its ubiq­uity caused it to fade into the back­ground. It doesn’t usu­ally even ap­pear in the web browser ad­dress bar any more. Whether dis­played or not, though, it is cru­cial to the world wide web, be­cause it de­fines how in­for­ma­tion is ex­changed be­tween web browsers and web servers.

In the thick mul­ti­lay­ered sand­wich of mod­ern com­put­ing, HTTP lies some­where in the mid­dle. It’s not re­spon­si­ble for shunt­ing raw data from one place to an­other over the in­ter­net – that job is per­formed by a sys­tem known as TCP/IP. Nor is it re­spon­si­ble for the ac­tual con­tent and lay­out of your browser win­dow – that’s the re­spon­si­bil­ity of the hy­per­text markup lan­guage (HTML) and its al­lies. HTTP bridges the gap be­tween TCP/IP and HTML.

HTTP as­sumes a con­nec­tion be­tween the browser and the server, and then it de­fines what re­quests the browser can make of the server and how those are ex­pressed.

When you click on a link your browser fol­lows the HTTP rules to ask the web server host­ing the site for the web page, file or what­ever the link points to. Even­tu­ally the server re­sponds and ac­cord­ing to the rules the first line of its re­sponse must con­tain a three-digit code fol­lowed by a brief text de­scrip­tion of what hap­pened. The text is for hu­mans, the code is for com­put­ers.

Codes in the 200s in­di­cate the re­quest was suc­cess­ful, while those in the 400s in­di­cate that there was some­thing wrong with it. Our friend 404 lives herein and in­di­cates the re­quested item wasn’t found on the server and the server doesn’t know where to find it.

Some­times you’ll see other codes in this range, such as 403 which in­di­cates unau­tho­rised ac­cess. This usu­ally hap­pens if you try to visit a pass­word­pro­tected page with­out the right

cre­den­tials. There are other cat­e­gories of re­sponse codes too: the 500s in­di­cate a server er­ror, and the 300s re­di­rect the browser to a new lo­ca­tion where the re­quested item now re­sides.

Re­sponse codes are hid­den most of the time even though ev­ery browser re­quest is an­swered with one.

When things go wrong, though, some­times the only thing to do is show the server’s re­sponse, and the code is part of that.

In­com­pre­hen­si­ble er­ror codes are dis­may­ingly com­mon on the net, though they all mean some­thing to some­one. Now 404 means some­thing to you. Alas knowl­edge isn’t al­ways power: know­ing what 404 means prob­a­bly won’t stop those dig­its get­ting you down next time you en­counter yet an­other bro­ken link.

Im­age: UFOTOPIXL10 - FOTOLIA.COM

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