Functional explanations for common network devices Part III

The third part of the popular explanation

　　The above terminology is a little bit more, the beginner may still have a little not understand the feeling of thorough. Below I will use these three kinds of network devices to make a popular analogy to help understand:

LAN is like a building, everyone (like the host) has its own room (the room is like a network card, room number is the physical address, that is, the MAC address, inside the person (host) a walkie-talkie, due to work on the same channel, so a person to speak, others can hear, this is the broadcast (send packets to all hosts) , only the target responds, while others hear but ignore (discard packets), and all the walkie-talkie devices that can hear the broadcast are enough to be a broadcast domain. And these walkie-talkie is a hub (hub), each walkie-talkie is like a hub on the port, we all know that the intercom can not listen to the speech, you must loosen the key to listen to, the same moment can only receive or hair work mode is Half-duplex. And the walkie-talkie at the same time can only have one person to hear clearly, if two or more people say together will have a conflict, can not hear clearly, so this constitutes a conflict domain.

[Broadcast domain]: a set of devices in a network. That is, all devices that the same broadcast package can reach become a broadcast domain. When one of these devices emits a broadcast, all other devices receive this broadcast frame. All ports in the hub and switch are in a broadcast domain, and each port on the router is a broadcast domain. ］

One day, the people in the building can't stand this inefficient communication, so upgrade the device, replace each person with an internal phone (switch switch, each phone is equivalent to a switch on a port), each has an internal number (the logical address is the IP address). (Here's an extra word for IP address and MAC address translation. The common two-tier switch identifies only the MAC address, it has a MAC address table built into it, and keeps and updates it to determine which port corresponds to the MAC address of that host, and the communications software we use is ip-based. IP address and MAC address conversion work, the ARP Address Resolution protocol to complete. In the beginning, no one knows which number corresponds to which person, so to call someone must first broadcast: "XXX, your number is." "My number is xxxx". So you have the target number, and all the inside numbers are added to the phone book (the Switch's MAC Address table) in this way, and the next time you can dial it directly to his extension instead of broadcasting it. We all know that the telephone is point-to-point communication equipment, will not affect others, the conflict is only the two ends of the speaker, so the entire large conflict domain is divided into a number of small conflict domain. Also, the phone can speak at the same time as the answer, and this mode of operation is Full-duplex. This is one of the reasons the switch is better than the hub performance.

[ARP (Address Resolution Protocol): ARP provides a dynamic mapping between the IP address (bit) and the corresponding hardware address (the value of the bit). Translation from the logical Internet address to the corresponding physical hardware address is the function of ARP. The key to ARP's efficient operation is that there is an ARP cache on each host. This cache holds the mapping record for the most recent Internet address to the hardware address. The lifetime of each item in the cache is typically 20 minutes, starting at the time it was created. This process is done automatically, and the general application user or system administrator is not concerned. ］

After a while, the people in the building want to contact the building next to it, so they pull a telephone line to the next floor (collectively, including DDN, optical fiber, Frame Relay and other connection methods). As we all know, dial 0 to the switchboard before dialing the destination number, the switchboard is the equivalent of a router. The number you dialed is also the number of the other switchboard, before you can dial the extension number to go to the destination extension. The switchboard is used to find each other's switchboard and to turn the intercom into an outside line number (NAT network address translation), which is the main purpose of the router. And such thousands of buildings are interconnected through intricate lines, forming the Internet.

[NAT network address translation translation: When an internal computer communicates with an external Internet network, network A device with a NAT capability is responsible for translating its internal IP address into a legitimate IP address (that is, the requested IP address) for communication. NAT refers to the process of this transformation. Broadband routers are based on NAT to enable multiple intranet hosts to share the Internet. ］

IP address has a private IP and public network IP points, like the same building inside the phone as long as you dial the inside number to communicate, the inside number can be customized, as long as in this building does not repeat can be, other buildings can also set the same number, do not worry about the building and building between the internal number of duplicate and conflict. This inside number is equivalent to private IP (in the TCP/IP protocol, some network segments are dedicated to private IP, these segments will not appear on the public network, such as: 192.168.0.0~192.168.255.255 this class C network segment, and the operator's outside line number is equivalent to the public network IP, the telephone office (like the ISP) to allocate.

[ISP (Network service provider): an abbreviation for Internet services provider, which is a vendor that can connect your computer to the Internet, and it can also provide e-mail, hard disk virtual space rental, hosting, and other network services. Our most frequently contacted ISP is the telecommunication, Netcom, the long width and so on broadband access provider. ］

Now, we all understand the use and difference of these network devices. As long as the corresponding selection of equipment according to the requirements, even the beginners of the network can correctly build their own local area network.