7．Your DNS cache （域名存储器）stores the locations （IP addresses）of web servers that contain web pages which you have recently viewed．If the location of the web server changes before the entry in your DNS cache updates，you can no longer access the site．
If you encounter a large number of HTML 404 error codes，you may need to clear your DNS cache．After you clear your DNS cache，your computer will query （询问）nameservers （域名服务器）for the new DNS information．The following methods allow you to remove old and inaccurate DNS information that may result in 404 errors．
Windows 7
To clear your DNS cache if you use Windows 7，perform the following steps：
Click Start．
Enter cmd in the Start menu search text box．
Right-click Command Prompt and select Run as Administrator．
Run the following command：
If the command succeeds，the system returns the following message：Windows IP configuration successfully flushed the DNS Resolver Cache．
Windows?8
To clear your DNS cache if you use Windows 8，perform the following steps：
On your keyboard，press Win+X to open the WinX Menu．
Right-click Command Prompt and select Run as Administrator．
Run the following command：
If the command succeeds，the system returns the following message：Windows IP configuration successfully flushed the DNS Resolver Cache．
Windows XP，2000，or Vista?
To clear your DNS cache if you use Windows XP，2000，or Vista?，perform the following steps：
Click Start．
On the Start menu，click Run．
If you do not see the Run command in Vista，enter run in the Search bar．
Run the following command：
If the command succeeds，the system returns the following message：Successfully flushed the DNS Resolver Cache．
MacOS?10.10.4 and above
To dear your DNS cache if you use MacOS X version 10.10.4 or above，perform the following steps：
Click Applications．
Click Utilities．
Click Terminal．
Run the following command：
| sudo killall-HUP mDNSResponde |
If the command succeeds，the system docs not return any output（输出信息）．Warning：
To run this command，you must know the computer's administrator account password．
25．If the command succeeds，all the following systems return a massage exceptB．
A．Windows?8
B．Windows 7
C．Windows XP，2000，or Vista?
D．MacOS?10.10.4 and above
26．What's the main purpose of writing the passage？B
A．To introduce something about several systems on computer．
B．To explain how to solve some problem in several systems on computer．
C．To explain how to access some sites in several computer systems．
D．To explain how to install DNS cache in several computer systems．
27．Which of the following is true about DNS cache，according to the text？A
A．DNS cache stores the IP addresses of web servers．
B．DNS cache needs to be cleared，if you meet a large number of error codes．
C．DNS information may result in 404 errors．
D．Unless the entry in your DNS cache updates，the location of the web server won't change．
