It's covered in the section titled "host name length": > A host name in a URL has no real size limit, but libcurl’s URL parser refuses to accept names longer t

It's covered in the section titled "host name length": > A host name in a URL has no real size limit, but libcurl’s URL parser refuses to accept names longer than 65535 bytes. DNS only accepts host names up 253 bytes. So, a legitimate name that is longer than 253 bytes is unusual. A real name that is longer than 1024 is virtually unheard of. DNS is not the only mechanism for resolving host name to address, even if it's what's used 99.9% of the time today.