  We explore the availability and persistence of URLs cited in articles
published in D-Lib Magazine. We extracted 4387 unique URLs referenced in 453
articles published from July 1995 to August 2004. The availability was checked
three times a week for 25 weeks from September 2004 to February 2005. We found
that approximately 28% of those URLs failed to resolve initially, and 30%
failed to resolve at the last check. A majority of the unresolved URLs were due
to 404 (page not found) and 500 (internal server error) errors. The content
pointed to by the URLs was relatively stable; only 16% of the content
registered more than a 1 KB change during the testing period. We explore
possible factors which may cause a URL to fail by examining its age, path
depth, top-level domain and file extension. Based on the data collected, we
found the half-life of a URL referenced in a D-Lib Magazine article is
approximately 10 years. We also found that URLs were more likely to be
unavailable if they pointed to resources in the .net, .edu or country-specific
top-level domain, used non-standard ports (i.e., not port 80), or pointed to
resources with uncommon or deprecated extensions (e.g., .shtml, .ps, .txt).
