A collection of networks that are under a single administrative control, like an ISP or a large organization, is called an Autonomous System.
在单一管理控制下的网络集合，如ISP或大型组织，称为自治系统。
>> Routers don't connect devices of the same network together.
>>路由器不把同一网络的设备连接在一起。
They connect different networks together.
它们把不同的网络连接在一起。
So you wouldn't ever see a PC connected to a router.
所以你永远不会看到PC连接到路由器上。
Connected to a router you'd find either a switch, or another router.
连接到路由器，你会发现一个交换机，或者另一个路由器。
That little box we have at home that everyone calls a router,actually has switched functionality built inside of it.
我们家里有一个小盒子，每个人都把它叫做路由器，实际上它内部已经有了交换功能。
If you've ever connected a desktop to an interface on that router,you actually plug the cable into the switch interface,even though we call that little box router.
如果你曾经把桌面连接到路由器的接口上，你实际上是把电缆插到交换机接口上，即使我们把它叫做盒子路由器。
As we've discussed earlier, when a router gets a packet to send to a destination IP address,the router consults its routing table to see if it has knowledge of the destination network,and if so, which router interface should get the packet next.
如前所述，当路由器收到要发送到目标IP地址的数据包时，路由器会查看其路由表，以确定它是否了解目标网络，如果知道，那么接下来应该由哪个路由器接口获得数据包。
If the router has no knowledge of the destination network,it might have a default route of its own to send the packet to.
如果路由器不知道目标网络，它可能有自己的默认路由来发送数据包。
Without either knowledge of a destination network, or a default route for a routerto send all packets with unknown destinations to, the router will drop the packetand send an error message back to the source through a protocol known as ICMP.
如果不知道目标网络，也不知道路由器发送未知目的地的所有数据包的默认路由，路由器将丢弃数据包，并通过称为ICMP的协议将错误消息发送回源。
Today's internet backbone routers are approaching three-quarters of a million,750,000 routes in the routing cables.
今天的互联网骨干路由器正在接近75万条路由电缆中的75万条线路。
On internal autonomous systems, there are much less.
在内部自治系统上，则要少得多。
An autonomous system represents a collection of networks under one administrative control,like an ISP, or major entity like RIT.
自治系统表示在一个管理控制下的网络集合，如ISP或RIT这样的主要实体。
Here at RIT, we have a collection of internal networks.
在RIT，我们有一系列内部网络。
Remember, a network refers simply to a collection of switchesand PCs between two router interfaces.
记住，网络仅仅是指两个路由器接口之间的交换机和pc机的集合。
Why would a bunch of networks be preferred to a single network?
为什么一组网络比一个网络更受欢迎?
Well, for one, think back to our arc stories, arc requestsand all other broadcast traffic will always reach and be processedby every single device on a network.
首先，回想一下我们的arc故事，arc请求和所有其他广播流量总是会到达并被网络上的每个设备处理。
One reason why we might want multiple networks interconnected by routers insteadof one big flat network is to reduce the size of the broadcast domain.
我们可能希望多个网络通过路由器而不是一个大的平面网络互连的原因之一是减少广播域的大小。
Instead of broadcasts tying up the bandwidth in processing, of all devices on a network,we've now lowered the amount of broadcasts that will proliferate through a networkand the amount of devices that can hear them.
我们现在已经降低了将通过网络激增的广播数量和能够听到广播的设备数量，而不是占用处理网络上所有设备的带宽。
It's why here at RIT each class is taught in a room by itself.
这就是为什么在RIT，每一节课都是在一个房间里自学的。
If we had one homogenous auditorium and had all classes there simultaneously,my students would have to listen and try to understand all the other professors and classes.
如果我们有一个同质的礼堂，所有的课程同时在那里，我的学生将不得不听，并试图理解所有其他教授和班级。
That would take their attention away from me and my lessons.
那会分散他们对我和我的课的注意力。
Another reason why multiple networks are preferredto one big flat network is for security purposes.
多个网络优于一个大平面网络的另一个原因是出于安全考虑。
Security at the router level in the form of an access control list,can be used to filter traffic by source IP address,destination IP address, protocols, and even ports.
路由器级别的安全访问控制列表的形式，可用于按源IP地址、目标IP地址、协议甚至端口过滤流量。
This allows you to control access to and from certain devices and resources much betterthan if everything was on the same network.
这使您能够更好地控制对某些设备和资源的访问和从这些设备和资源的访问，而不是将所有东西都放在同一个网络上。
It's also a way to hierarchically design an internetworkand even makes the troubleshooting process easierby isolating traffic to a certain network
这也是一种分层设计internetwork的方法，甚至可以通过隔离到特定网络的流量来简化故障诊断过程
转载于:https://www.cnblogs.com/sec875/articles/10358038.html
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