问题 单项选择题

在IPv4中,组播地址是()地址。

A.A类

B.B类

C.C类

D.D类

答案

参考答案:D

解析:

IP组播(也称多址广播或多播)技术,是一种允许一台或多台主机(组播源)发送单一数据包到多台主机(一次的,同时的)的TCP/IP网络技术。组播作为一点对多点的通信,是节省网络带宽的有效方法之一。在网络音频/视频广播的应用中,当需要将一个结点的信号传送到多个结点时,无论是采用重复点对点通信方式,还是采用广播方式,都会严重浪费网络带宽,只有组播才是最好的选择。组播能使一个或多个组播源只把数据包发送给特定的组播组,而只有加入该组播组的主机才能接收到数据包。目前,IP组播技术被广泛应用在网络音频/视频广播、AOD/VOD、网络视频会议、多媒体远程教育、 push技术(如股票行情等)和虚拟现实游戏等方面。

IP组播通信必须依赖于IP组播地址,在IPv4中它是一个D类IP地址,范围为 224.0.0.0~239.255.255.255,井被划分为局部链接组播地址、预留组播地址和管理权限组播地址三类。其中,局部链接组播地址范围为224.0.0.0~224.0.0.255,这是为路由协议和其他用途保留的地址,路由器并不转发属于此范围的IP包:预留组播地址范围为 224.0.1.0~238.255.255.255,可用于全球范围(如Internet)或网络协议:管理权限组播地址范围为239.0.0.0~239.255.255.255,可供组织内部使用,类似于私有IP地址,不能用于Internet,可限制组播范围。

单项选择题 A3/A4型题
阅读理解

I received pictures of my daughter floating (漂浮) on a lake in California. Right there under the sun during the hottest time of the day, she has only her swimsuit to cover her.

When I showed it to my students, it was as if I had shown them scenes from a horror (恐怖) movie.

“But she will get dark,” they said.

“That’s what she wants,” I told them. “We think darker skin (肤色) is beautiful.”

They looked at me in disbelief. It was as if I came from the moon.

For years, American girls spend their summer trying to get as tanned (晒黑) as possible. In China, on the other hand, girls use umbrellas all year round.

What in the world is going on?

The pale skin we think looks unattractive (不漂亮) is what Asian women want. Both groups want to meet their culture’s standard (标准) of beauty.

In the US, if you can afford to go on vacation in the summer, you head for a river, a lake or a beach, where you try to get as much sun as possible. If you are rich you take a winter vacation in some sunny spot. Your winter tan shows your wealth.

In China, once upon a time, the few wealthy were the ones who didn’t have to work out in the hot sun. So the whiter the skin, the richer the woman. The folk opera performers who play these ladies wore lots of white makeup (妆容). The tradition survives to this day.

So it comes naturally that none of my students wishes to follow the Western ways in this regard.

But now I am noticing that a new business – the tanning booth (美黑店) – started up in China. I admire those brave enough to define(给…下定义)their own idea of beauty, but I do wonder what their grandmothers would have to say to them about it!

小题1:The students were shocked at the picture the writer showed them because ______.

A.they thought it shameful for a girl only to wear a swimsuit

B.the writer’s daughter was wearing very little while out in the sun

C.it is dangerous for a girl to be out on a lake alone

D.with the powerful sunlight the writer’s daughter would have got a suntan小题2: According to the article, which kind of woman meets the American standard of beauty?

A.Women with blond hair.

B.Women with a slim figure.

C.Women with tanned skin.

D.Women with pale skin.小题3: In the US a winter tan usually means the person _____.

A.is hard-working

B.doesn’t have to work

C.has a lot of money and spare time

D.is from a very poor family小题4:The writer ______ the tanning booth started up in China.

A.is not interested in

B.doesn’t understand

C.doesn’t like

D.has mixed feelings about