hostip.c explainedhost*.c source file are these:CURLRES_IPV6getaddrinfo() and family, and thus we use that. The host may not be able to resolve IPv6, but we do not really have to take that into account. Hosts that are not IPv6-enabled have CURLRES_IPV4 defined.CURLRES_ARESCURLRES_THREADEDCURLRES_ASYNCH is defined too. If libcurl is not built to use an asynchronous resolver, CURLRES_SYNCH is defined.host*.c sourceshost*.c sources files are split up like this:hostip.c - method-independent resolver functions and utility functionshostasyn.c - functions for asynchronous name resolveshostsyn.c - functions for synchronous name resolvesasyn-ares.c - functions for asynchronous name resolves using c-aresasyn-thread.c - functions for asynchronous name resolves using threadshostip4.c - IPv4 specific functionshostip6.c - IPv6 specific functionshostip.h is the single united header file for all this. It defines the CURLRES_* defines based on the config*.h and curl_setup.h defines.