Name: Default Gateway Explained
Uploaded: 2022-12-13T16:37:47.000Z
Duration: 6 min 35 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

What is a default gateway?  So that is the topic of this video.

Now as a demonstration on a Windows computer, let's  check the network configuration.  So if you open up  

a command prompt and then you type in ipconfig  and in the output you'll see the IP address,  

subnet mask, and the default gateway that's  been assigned to this computer.  So you might  

be asking yourself, well what is a default gateway? And simply put, a default gateway is a device that  

forwards data from one network to another.  And the  majority of the time, this is going to be a router.  

So for example here we have a local area network.  So here is the router, switch, and the computers.  

And on the other side of the router we have the  internet, which is another network.  So in order for  

these computers to access another network, such  as a web page out on the internet, the data has  

to exit its own local network by going through the  default gateway, which is the router.  And then the  

router will forward the data to the internet.  Now  this also works both ways.  So if a device on the  

internet wanted to communicate with a computer on  this network, it has to go through this network's  

default gateway and then to the computer.  So  in a nutshell, that's what a default gateway is.  

It lets devices from one network communicate with  devices on another network.  And as I said before,  

this is typically going to be a router.  A router  is the gateway or doorway to every network.  And 

the term default means that the designated  device is the first option that's looked upon  

when data needs to exit the network.  Now if these  computers here wanted to communicate with each  

other, they can just talk directly to each other  through the switch.  And this is because all these  

computers are on the same network.  Their data  doesn't have to exit the network and go through  

the default gateway.  So this brings us to our next  question.  And that is, if these computers wanted to  

communicate with another computer, how do they know whether that computer is on their own network  

or if it's on a different network.  Because as I stated  before, if this computer wants to communicate with  

a computer on the same network, it can just talk  directly to it.  But if it wants to communicate  

with a computer on a different network, it has  to go through the default gateway.  So again how  

does it know?  And this is where the IP address and  subnet mask come in.  An IP address consists of two  

parts.  The first part is the network address and  the second part is the host address.  So the way to  

tell which portion belongs to either the network  or the host, is where the subnet mask comes in.  

A subnet mask is a number that resembles an IP  address.  And it reveals how many bits in the IP  

address are used for the network by masking the  network portion of the IP address.  So here we have  

the IP address and subnet mask in binary form.  So the way to tell which portion of this IP address  

is the network portion, is when the subnet mask  binary digit is a 1 it will indicate the position  

of the IP address that defines the network.  So   we'll cross out all the digits in the IP address  

that line up with the 1s in the subnet mask.  And  when you do this, it will reveal that the first  

three octets or sets are the network portion  and the remaining is the host portion.  So any  

computer or device on a network where the first  three numbers of the IP address are 192.168.0  

means that those computers are on the same network.   Which means that the computers can talk directly  

to each other without exiting through the default  gateway.  And then the host portion is what's  

uniquely assigned to devices, such as computers.  So here we have a private network that has  

been divided into two sub networks or subnets.  The  subnet on the left is on the 192.168.0 network and the  

subnet on the right is on the 192.168.1 network.  And each subnet has their own default gateway.  

Now let's say that computer A wanted to  communicate with computer B on this subnet.  

So computer A is going to check computer B's IP  address to see if it's on the same network or not. 

And as you can tell, the two computers are on the  same network because the network portion of the IP  

addresses, which are the first three octets, are the  same.  So computer A now knows that computer B is on  

the same network.  So now in order for communication to take place, computer A needs computer B's MAC 

address.  And it finds this by sending out an ARP  broadcast out on the network asking computer B  

for its MAC address.  Then once it has the MAC  address, communication can finally take place.

So in another scenario let's say that computer  A on this subnet here wanted to communicate with  

computer D on this subnet.  So again computer  A is going to check computer D's IP address  

to see if it's on the same network or not.  And  as you can tell this time, the two computers  

are on different networks because the network  portion of the IP addresses, which are the first  

three octets, are different.  And the difference  is the third number.  Computer A is using a 0 

and computer D is using a 1.  So computer A now  knows that computer D is on a different network.  

So it can't directly communicate with it, it has  to use the default gateway.  So computer A will  

send out an ARP broadcast and this time it'll ask  for the MAC address of the default gateway and not  

the computer, because computer D is on a different  network and it won't receive the broadcast because  

ARP broadcasts cannot go past a router.  Then once  it has the MAC address, it'll send the data to the  

default gateway and then it'll be forwarded to the  destination.  So everyone that concludes this video  

on default gateways.  Thank you for watching, please  subscribe, and I'll see you in the next video.