Network Connection Status Indicator (NCSI)

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… Windows machines try to request two URLs (www.msftncsi.com/ncsi.txt and ipv6.msftncsi.com/ncsi.txt, the former over IPv4, the latter over IPv6) to ascertain whether a given network is routed to the Internet and if there is a captive portal in the way (NCSI stands for “Network Connection Status Indicator”).

Ref.: http://arstechnica.com/information-technology/2015/08/even-when-told-not-to-windows-10-just-cant-stop-talking-to-microsoft/

See also: https://blogs.technet.microsoft.com/networking/2012/12/20/the-network-connection-status-icon/.