Why do Android devices send GET requests to Google IP addresses like /generate_204?

Question

Google

Content filtering by the registry of prohibited sites in one provider. In addition to URLs and domains, domains in the form of IP addresses were also slaughtered. But it was noticed that some android devices access Google like this:
173.194.32.164/generate_204
173.194.32.163/generate_204 173.194.32.166/generate_204 There are more
addresses
, only a small part is higher. I tried to contact, got: HTTP/1.1 204 No Content.
Is this a normal situation? Why deal with IP as Host? What happens if you slaughter the data of the appeal? What exactly makes such requests? And why?
PS stackoverflow.com/questions/1989214/google-com-and... - there is a similar question where there are many suggestions. However, in those cases, the appeal still occurs by domain names.
PPS I made a dump on my home router. Address 1.1.1.1 - router, 1.1.1.3 - phone. First, it makes a DNS query for a specific domain, then accesses it by IP. This happens every time after connecting via Wi-Fi. Later I killed any packets where "generate_204" (iptables string) occurs, I did not notice any changes on the phone. Sample dump: https://dl.dropboxusercontent.com/u/53468586/toste...

Answer 1

This is an attempt by Google to understand if you need to log in to the network or you can surf just like that. That is, if you go to McDonald's, then Google will send this request, get nothing and say "you need to log in."

Answer 2

www.gstatic.com/generate_204
www.gstatic.com