Head first Network部分笔记

1. Patch Panel :
什么事Patch Panel? 有什么用？
A patch panel is used to organize the cable and wire connections in your wiring closet. Cables comming from your network devices or wall jacks plug into the back of the patch panel, and these are connected to other devices using short patch cables at the front. It's typically mounted on a network rack, a specialized frame that forms the physical skeleton of your network closet.
A patch panel works like a telephone swithboard, but for cables.


2. Hub 集线器 无MAC地址
A hub is sometimes called a repeater because it repeats the incoming signal using no digital intelligence such as memeory or a processor.
It simply repeats incoming signals on all ports, without making any changes to the signal before broadcasting it.
Hubs don't change the MAC address. A hub sends signal, and sends them everywhere.

3. Switch 交换机
A switch sends frames, and only sends them where they need to go. Switches avoid collisions by storing and forwarding frames on the intranet. Switches are able to do this by using the MAC address of the frame. Instead of repeating the signal on all ports, it sends it on to the device that needs it.
A switch reads the signal as a frame and uses the frame's information to send it where it's supposed to go. Switches are smart.
Switch stores MAC addresses(MAC address tables)

4. Router 路由
Routers have MAC addresses too. If network traffic comes from a router, we can only see the router's MAC address. All the workstations behind that router make up what we call an IP subnet（子网）. All a switch needs to look at to get frames to their destination is the MAC address. A router looks at the IP address from the incoming packet and forward it if it is intended for a workstation located on the other network.
Routers are really smart.


5.Q: So why do we have to use a router? Can't the switch connect to the radio?
  A: We have to use a router when we connect two networks. The router acts as a 'translator' between the two networks. Simple switches do not have the brains for this.(p226)

Q: What is the router "translating"?
A: In simply terms: network addresses. The two different networks are like 2 different cities. The router moves the data from one network to antoher.

Q: So do I always need to connect a computer to a switch?
A: To a switch or to a hub, but NEVER directly to a router.

Q: But I have a DSL router at home, and my computer is directly connected to it. What is that all about?
A: Good observation观察. There are switches that have routing capability and routers that have switched ports. There is not a real clear line between the two devices. It is more about their primary function. Now, in large networks, there are switching routers. There have software that allow them to work as routers on switched ports. They are great to use and make building large sophisticated network straightforward, but they are very expensive.


6. Only network nodes with the same IP network address can communicate across a switch.
A switch can only deal with MAC addresses. A router is needed to connect two different IP networks.

So why can't a MAC address be used to move traffic from one network to another?

7. Ip addresses give our networks a sence of location, and network nodes a sense of belonging to that location.

8. ARP
Address Resolution Protocol地址解析协议
Find out the MAC addresses by using the IP addresses. It does by using ARP.

9. Q: I thought I just needed a MAC address for a frame to get somewhere.
   A: THat's all an Ethernet frame needs to get somewhere. But, remember from the previous chapter that Ethernet frames contain things called packets, and these contain data in the form of protocols. In the case of TCP/IP networks, we need IP addresses to move the packets between networks.

   Q: If my computer has an IP address, why does it need a MAC address too?
   A: A computer can talk multiple network protocals on the same network. e.g, on my Mac here, it is talking TCP/IP and Appletalk on the same Ethernet line. So the MAC address allows the Ethernet frames to move between network devices like routers and switches. The network protocols allow the computer to talk to devices on the other networks.


10. Network devices have a default gateway in their network setup. This is the router's IP address. It is where the devices send all network traffic bound for other networks.

11. Remember, the only addresses the switch understands are MAC addresses.
An ethernet switch just looks at the MAC address of a frame and forwards that frame to the correct device. It does not modify the frame in any way.

The router has to actually take the packet out of the frame, get the IP address, then modify the frame MAC address if it needs to send it on to a device on another network.


12. router table???
http://en.wikipedia.org/wiki/Routing_table

13. The switch decides where to send traffic based on the MAC address.
    THe router decides where to send the traffic based on the IP address.
14. Remember, the only addresses the switch understands are MAC addresses.
An Ethernet switch just looks at the MAC address of a frame and forwards that frame to the correct device. It does not modify the frame in any way.
The router has to actually take the packet out of the frame, get the IP address, then modify the frame MAC address if it needs to send it on to a device on another network.

15.http://loveme23.blog.51cto.com/

16. Routing tables tell routers where to send packets.

17. Use RIP to get routes to update themselves.
RIP Routing Information Protocol is a way for routers to share network addresses. Routers use RIP to talk to each others, sharing their route information and allowing them to keep their tables up-to-date.

18. The Hop-Count is the number of routers a packet must "hop" through to get to a particular IP network. RIP can only use the hop count to find the best route. RIP doesn't know the speed of a particular network line,  so it treats all lines equally.

19. how to choose the protocal for
RIP, Static Route Tables,  OSPF, EIGRP page293.

20. the domain name system. page305

21. SNMP(Simple Network Management Protocol) is a way to talk to your network devices and get all kinds of information from them without having to connect each device to a laptop.

22.DHCP Dynamic Host Configuration protocol
DHCP allocates IP addresses. DHCP makes our life easy by offering to clients IP addresses it has in its pool of available addresses.

First make sure the client has DHCP turned on...
选中：自动获得IP地址 即可

second make the wireless access point a DHCP server...


23. NAT Network Address Translation
NAT works by reallocating IP addresses. P393

24. oracle_pl_sql_programming_5th_edition.pdf