code: func (c *Client) serverNameOfAddr(addr string) string {
  	serverName, _, err := net.SplitHostPort(addr)
  	if err != nil {
  		c.printf("could not get server name from address %q: %v", addr, err)
  		return ""
  	}
  	if ip := net.ParseIP(serverName); ip != nil {
  		return ""
  	}
  	return serverName
  }
docs: // serverNameOfAddr returns the host part of addr, or the empty string if addr
  // is not a valid address (see net.Dial). Additionally, if host is an IP literal,
  // serverNameOfAddr returns the empty string.