I have noticed several posts with links to pdf files where IP address is used instead of domain name the url.1 (I will list them below.) Several of them seem to be dead. Moreover, this type of url seemed unusual to me. So I wanted to ask a few thinks about them:

• Do they work for other users? (When I tried the links, I did not get the files. But it's possible that I am the only person with the problem.)
• Is it possible to say whether they worked at the time when they were posted? Is it possible that the links work (or worked) only for some users?
• Are there some advantages to posted such links rather then using the more "usual" form? Is there some plausible explanation why somebody might have posted such links by mistake?

Here are the links that I found:

1I was playing around with checking MO posts for dead links. This was based on YCor's suggestion in a comment to another question. Originally I only checked jpg files, at that time I posted some details about what I triead in chat. Recently, there was also a discussion about archiving externals links from posts on SE sites on Meta Stack Exchange: Automatically have links archived in the Wayback Machine.

• 129.81.*.* is Tulane University. 130.44.*.* is AMS. 130.220.*.* is University of South Australia. 131.220.*.* is Uni-Bonn. 129.69.*.* is Uni-Stuttgart. 129.187.*.* is Leibniz-Rechenzentrum (Dagstuhl?), 146.163.*.* is Southern Illinois University Edwardsville. – François G. Dorais Aug 15 '19 at 7:47
• @FrançoisG.Dorais That seems likely, considering that the link dauns.math.tulane.edu/~vhm/papers_html/final-bell.pdf does work. Still, the urls starting 129.81.170.14 do not work (at least for me). BTW it's good to see you back. – Martin Sleziak Aug 15 '19 at 7:49
• I edited one of the Tulane ones, where a commenter pointed out the arXiv reference of the paper, and the author of the answer (and the paper) agreed that that would be a better link, but didn't edit. – David Roberts Aug 15 '19 at 7:50
• Moderators have the advantage that they can edit the comments even after the grace period. Quite sneaky. :-) – Martin Sleziak Aug 15 '19 at 12:21
• As one of those who unwittingly posted a PDF link beginning with an IP address, I would like to know "Is there some plausible explanation why somebody might have posted such links by mistake?" In my case, I was just copying the URL in my browser that brought me the PDF. – Joseph O'Rourke Aug 15 '19 at 12:54
• I took the liberty of fixed a few more, and noting which ones in the OP. – David Roberts Aug 16 '19 at 0:46
• Now only ten starting with 129.81.170.14 and two with 131.220.77.52. I can slowly edit these over the next week. – David Roberts Aug 16 '19 at 7:59
• Six left URLs left starting with 129.81.170.14. I'm taking the liberty of improving the references where possible, since opaque see this link references force users to click through merely to see if they already know what is being linked to. – David Roberts Aug 19 '19 at 0:38
• OK, all the problematic ones are now fixed. I'm not going to go through the ones that currently work, but it might be an idea to slowly convert these to non-IP address links. – David Roberts Aug 21 '19 at 0:04

Do they work for other users? (When I tried the links, I did not get the files. But it's possible that I am the only person with the problem.)

I think they aren't user-dependent; they are just rather short-lived. After all, they are IP addresses.

Are there some advantages to posted such links rather then using the more "usual" form?

No, only disadvantages (they die quickly, and web.archive.org doesn't archive them even if you request it to).

Is there some plausible explanation why somebody might have posted such links by mistake?

I'd guess it's because of misconfigured (university) servers.

• This is only tangential, but looking at this post (current revision), it seems that also "IP-based" url can be added into Wayback Machine. Here is the direct link. – Martin Sleziak Aug 21 '19 at 3:41
• I have just tested this with the link to dieter.pdf and it was possible to add it. (One of the working links in the question.) I have added it using http://web.archive.org/save/[url].) – Martin Sleziak Aug 21 '19 at 3:41
• @MartinSleziak Oh, then I must have hit some other problem. Thanks for doing this! – darij grinberg Aug 21 '19 at 5:31