What is the role of the computer host gateway?

Reprinted from "What is the function of computer host gateway"

Suppose your name is small, you live in a large yard, your neighbors have a lot of little friends, in front of the door and a look at the door of Li, Uncle Li is your gateway. When you want to play with a little friend in the yard, as long as you shout his name in the yard, he hears it, responds to you, and runs out to play with you.

But you are not allowed to go out of the door, you want to contact with the outside world, all must by the door of Uncle Li (Gateway) to help you contact with the phone. If you want to find your classmate Xiao Ming chat, Xiao Ming lives in a far away in another yard, his home yard also has a janitor Wang (Xiao Ming's Gateway). But you do not know the phone number of Xiao Ming's home, but your teacher in charge of your class has a list of all the students and telephone number comparison, your teacher is your DNS server. So you at home to dial the door Li uncle's phone, with the following dialogue:

Little: Uncle Li, I would like to find a teacher in charge of Xiao Ming's telephone number line?

Uncle Li: Ok, you wait. (then Li uncle to your teacher hung a telephone, asked clearly Xiao Ming's telephone) asked, his family's number is 211.99.99.99

Tiny: Great! Uncle Li, I want to find Xiao Ming, you can help me contact Xiao Ming.

Uncle Li: No problem. (then Uncle Li to the telephone office to send a request to connect Xiao Ming's home phone request, the last of course is transferred to Xiao Ming's home that yard Wang uncle there, and then Wang uncle put the phone to Xiao Ming's home)

So you get in touch with Xiaoming.

As for the DHCP server, it can be likened to:

There are more and more residents in your yard, the front of the front of the telephone switch is not enough to meet the needs of so many residents, so had to adopt a new technology called DHCP, the residents on the start of a random phone number, every time the number may be different

Your door's big Uncle Li: Is your gateway

Your head teacher: That's your DNS server.

The phone switch in the front: your DHCP server

Ditto, the dialogue between Uncle Li and Wang Ye is called routing.

Another: If there is a child called the small dark, he lived in the yard of the janitor is sun, because the small dark yard just cover, Sun Uncle just came, he did not Li and Wang Uncle's office telephone (Li and Wang Ye of course did not his phone), then there will be two situations:

1, the neighborhood of Zhao Aunt told Sun Big Uncle about Li, Wang two uncle's phone (at the same time Zhao aunt also told Li Wang about Sun's phone), this is called static set routing

2, Zhao is sick, uncle Sun himself around the phone, see people said: "I was a small dark their yard tube phone", the result was Lee, Wang two heard, in their address book, and then Lee, Wang to the Sun master back a telephone said: "I am Xiao Ming (small) Their yard tube phone", this is called dynamic set routing

Then one day little to find small dark, the result is naturally tiny to Li uncle Call said: "Uncle, I find small dark" (here omitted to check the process of small dark phone, assuming he knew the small dark phone), Li uncle a Find Address book: "Oh, the small dark yard phone is Sun Uncle Guan, To find a small dark nature first notice, I can inform Wang uncle let him go to find sun uncle, also can himself directly to the sun, that is of course, he is directly to the sun convenient ", so uncle Li to Sun uncle called, and then sun Big Uncle again put the phone to the small dark home."

Here, Uncle Li's address book is called the routing table.

Mr. Li chose to be his own direct to the Sun master or let Wang Uncle help transfer called route selection.

Uncle Li chose to directly find Sun Uncle is based on, because he directly find Sun uncle can one step, if you want to Wang Uncle transfer will need two steps to complete, here "step" is called "Hop number", Uncle Li's choice is to follow the minimum step (hop number) principle (if he does not abide by this principle, A little bit may be more time to find the small dark, the end result may be caused by the failure of Li's job was fired, which is called "delay too long, the principle of routing unreasonable, changed a router")

Of course, things are always changing, small and Xiao Ming quarrel, these days the little boy always give a phone, Xiao-Ming thought: ", he is not talking about my bad ah?" So Xiao Ming decided to eavesdrop on small and little dark calls, but he can not out of the yard, how to do? Xiaoming made such a decision:

First he told himself in the hospital phone King Uncle said: "You give Uncle Lee call said small dark moved to our yard after all calls to his phone I come to pick up", Uncle Wang did not reflect (after all, old Ah!) He called Uncle Li, said: "Now I come to manage the small dark phone, sun has no matter", the results of his uncle Li's address book changed, this is called routing deception.

After small little again to find the dark, Lee Big Uncle to Wang (in fact should be transferred to the master), Wang Uncle received this call to Xiao Ming (because he had already and Xiaoming agreed), Xiao Ming received this phone pretend small dark and tiny communication. Because Xiao Ming Zuozeixinxu, afraid of Tomorrow little and small dark meet after the face to ask him, then the communication broke, and his own in the name of small little dark through a telephone repeated the words just now, there is this is called data eavesdropping.

Then, the little one is constantly and small dark contact, and the snub of Xiao Ming, Xiao Ming's heart muttered AH: "I can not always so small dark identity and little talk Ah, outside a day out of what to do!" "So he thought of a more insidious trick:" I don't even listen to your phone, your little one not to call me! Then I'll let you give the little dark to play, hum! "And how did he do it?" Let's see:

He contacted a group of friends, and they colluded well, every day fixed a time everyone to the small dark yard to call the front, the content of everything, as long as the front of the sun grandfather answer the phone, you will hear "thunder, rain to accept clothes ah!" "," The person is the mother born, the demon is the demon his mother born "," Your mother's surname "and so on, listens to the head to be big, does not listen and not, the telephone keeps ringing! Finally one day, Grandpa Sun can't help, shouted: I can't stand pull!!!! "And hanged himself!"

This is the simplest DDoS attacks, Grandpa Sun's weak psychological ability to withstand the phenomenon is called "Data processing module has a bug", Grandpa Sun's suicide is called "router paralysis." If it were me, I would smile and small talk with them, such as telling them, "I have heard the weather forecast, the clothes have been collected 10 minutes ago" or "that your mother is a person or demon" or "with your grandmother a surname" and so on, I this sound psychology is called "robust datagram processing, can withstand any attack".

After his grandfather collapsed, the little one finally no longer to the small dark phone, because no matter how he hit each other is a busy tone, this phenomenon is called "denial of service", so Xiaoming's practice also has a name called "Denial of service attack".

Xiaoming was finally quiet for a few days, 、...

A few days later, Xiao Ming's yard came a beautiful girl, named Small Li, Xiao-ming very much like her (little age to play what Puppy love!). But Xiao Li has a very handsome boyfriend, Xiao Ming despair no way. Of course, it is still to follow the above principle: Xiao Li is not allowed out of the yard. The man wants to soak up Xiao Li nature can only call, so Xiao Ming again stirring:

Do you remember Grandpa Wang is the phone director of the yard? He can manage the phone because he has an address book, because the same yard may have 2 children are called Xiaoming, by the name can not be distinguished, so there are only two lines in the Address book

House phone

Gate 1234567 (This is Xiao Ming's)

Gate Second, 7654321 (this is Xiao Li's)

Grandpa Wang has a bad memory, but this is always not wrong (the same yard will not have 2 "gate second" bar)? Every time I call someone to say to find the phone number, and then through the address book to the door to knock, such as people say I find "1234567", so Wang grandfather a comparison, oh, is a door, he went to knock a door "listen to the phone", if it is to find "7654321", then he looked for the door second

The phone number here is the legendary "IP address".

The house number is the legendary "Mac" address of the network card (each NIC's MAC address is not the same, this is the network card manufacturer wrote dead in the chip of the network card

Xiao Ming thought "Grandma's, I can not soak you do not want to bubble", so he played the king's grandfather Address book idea, after careful observation, careful preparation, he finally found that Grandpa Wang has frequent urination problems (after all, the old man ah ...) , finally in a month of black and high day, Grandpa Wang went to the toilet, Xiao Ming secretly touched into the front, carefully changed the Wang Grandfather's address book ...

After a few days, Xiao Li's boyfriend again to call Li, the other party reported the phone is "7654321", Wang Grandpa a look at the address book, by:

House phone

Gate 1234567 (This is Xiao Ming's)

Gate 7654321 (Note: This is Xiao Li's, but was changed by Xiao Ming)

Grandpa Wang did not know changed Ah, so he went to find a door small clear, Xiao Ming in the heart of this beautiful ah, he to Xiao Li father's tone of the harsh lesson of the man and Xiao Li between the improper relationship between men and women, the result of the man's respectfully hung up the telephone. Of course, Xiao Li did not know the whole thing happened ...

Here Xiaoming's behavior is called "ARP spoofing" (because it is implemented by sending ARP packets on the actual network, so called "ARP spoofing"), Grandpa Wang's address book is called "ARP table"

Here to note: Grandpa Wang now has two contacts, one is to record each yard of the telephone book, called "Routing table", one is now said to record the details of the yard, called "ARP table."

There is a famous saying that "people are always in pursuit of perfection, even though they can never do it" (remember this phrase, because it's a big celebrity – that's what I say)

One of Grandpa Wang's system says, "to re-examine the house number and the phone's counterpart (that is, the ARP table) every month, this action is called" Refresh arp table "The time limit for each month is called" Refresh ARP table cycle ". So Xiao Ming in order to let that man can never find Xiao Li, and then every month to secretly change the address book, but this is also have to do!

To add, Xiaoming is very clever, if the Address Book (ARP table) is changed to this:

House number (MAC) phone (IP)

Gate 1234567 (This is Xiao Ming's)

Gate Second 1234567 (note: This was changed by Xiao Ming but he was dizzy and wrong)

will be the computer will pop up a dialog box prompting "Duplicate IP address", will eventually lead to Grandpa Wang at a loss, so inform the door and gate second, your phone repeated. So little Li knew someone was destroying her good, and this phenomenon was called "the scam was revealed."

Little knew Xiao Ming overheard him and small dark telephone, so and small dark agreed to the password. The little boy told the dark after encrypting what he wanted to say at home. Potato-〉 Wednesday, sweet potato-〉 treat, fool-〉 little home. So the little boy told the Dark: Potato fool sweet potatoes. Xiao Ming listens??? Don't know ... Depressed ... This is encryption.

In addition, Xiao Li also knew that Xiaoming changed his home telephone number. So grandpa Wang went to one of the calls and the house number. and hide not allow outsiders to modify, can only have their own keys (password). This is the IP address and the MAC address binding. When someone changed their phone number, they had to look for grandpa Wang to change it. The trouble is trouble, but it's safe. But Xiao Ming secretly stole the key of Grandpa Wang with a (theft password success), so he can also modify. That's it, that's it.

Good good Study, day.

Sequential Selection Cycle Summary

What is the role of the computer host gateway?