Why the links with "leagues/{site-name}" do not work for all sites?

Question

Typically when you go to reputation league, the url looks something like https://stackexchange.com/leagues/{id}/{site} or https://stackexchange.com/leagues/week/{id}/{site}, etc. The link contains some numerical value followed by site name, for example https://stackexchange.com/leagues/1/stackoverflow.

Since those numerical ids of sites can be changed, when posting a link to some information league, the links of the form https://stackexchange.com/leagues/{site} might be preferable. And they indeed work (you get redirected to a longer url including the "id"), some examples: Stack Overflow, Super User, Arqade, English, Ask Different, Server Fault,... However, the shorter version does not work for MathOverflow - you need to you a longer url which includes the numerical value - which is currently 504.

Question. Why the "shorter" link does not work on some sites? Is MathOverflow the only site where the shorter link does not work?

Answer 1

Try this: https://stackexchange.com/leagues/mathoverflow.net

The "site name" parameter is actually the host name - so Super User is https://stackexchange.com/leagues/superuser.com, English is https://stackexchange.com/leagues/english.stackexchange.com, etc. But that's verbose, so the route accepts shorter names and tries to guess the rest of the host name:

return SiteByHost(sitename) 
    ?? SiteByHost(sitename + ".stackexchange.com") 
    ?? SiteByHost(sitename + ".com");

Ah... But MathOverflow's host is mathoverflow.net - and since no other sites have a .net host, nobody thought to try completing the partial name with ".net". Thus, to get to MathOverflow's league, you must specify the entire host name.

Update: as of May 23, 2019 .net is also checked as a possible domain option.