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(URL means Universal Resource Locator, or simply Internet address.)
Sometimes, one of these URLs will not respond. If this happens, try again a few minutes or a few hours later.
Sometimes, a URL will simply disappear, and all links on the Internet that point to that URL immediately become invalid. In such cases, which are not rare, the owner of each of these links will find out about his/her broken link the next time he/she checks that link by clicking on it -- some owners will find the broken link in a few days, others may not find out about it for weeks or even months.
"Estimates put the average lifetime for a URL at 44 days": Scientific American, volume 276 number 3, March 1997, page 83.
Sometimes, the URL for an Internet page will be changed by the owner. When this happens -- after about 44 days on average, according to Scientific American, -- sometimes the owner will leave a temporary forwarding address at the old location, which makes it easy for anyone to make the appropriate correction in a link. Sometimes a URL will be changed but no forwarding address is set up at the old location. Sometimes a page is simply deleted and its URL disappears from the Internet.
When I first set up this site, in February 1996, I checked each link every two weeks, and corrected or deleted those that had stopped working. As the site grew larger, and the number of links increased, it was no longer possible to check every link every fourteen days. The checking interval -- established by setting my browser option for the expiry time of followed links -- was increased to 21 days, which was okay for a couple of months. Then I had to increase the checking interval to 28 days, and finally 35 days.
Now I have no set interval. I spend about 20 to 25 percent of my on-line time checking links, each time choosing one of the pages with the oldest links. I suspect most owners of web sites have a similar method of checking their links.
Of course, all of the above applies to links to external pages. Links to internal pages, that is to locations within my own site, are kept up to date at all times.
Latest revision: 1997 March 06