IPv4 Loopback and APIPA transcript

We are going to look into two different addresses. One is the loop back address. Another is the link local address. Now um loop back address loop back address allows a device to communicate with itself. What does it mean? It means if you are sending a ping the ping will come back to you. But how it is possible? Because if you are sending a ping to 8.8.8 8 it actually goes to 8.8.8 state. But how can you ping yourself like uh you studied private IP address range?

There is another range. This is the range. So if you look at here, this is the range. So this portion and this portion look at here. So any number okay is in this range is a loop back address. The concept of loop back address goes beyond this. Like for example, you can have a loop back address on a router and you can set any number. Let's say like for example you can you can create this 8.8.8 as a loop back as well on a router if you want.

But uh we are not going into those kind of things. Okay, just simple term. So if you open your computer cmd and if you do ping any number 127 dot any number dot any number dot any number you'll see it will you will get a positive reply. That means the ping is successful. Why is that? The reason is um you are pinging yourself and you are you are alive. Okay. So this is the most commonly used but you can use anything. So you might say if I have this can I ping as well.0000.

Yeah. So any number you ping from here at any point in time you'll get a positive. They all are part of the loop back. Okay. So you are pinging yourself which is called the local host. So sometimes you will see um while testing a website or something people they will type local host to see if there for example Apache engineext and those servers are working or not. So instead of local host you write local host in the browser instead of local host if you type 127.0.0.1 is the same thing.

Now can I type here 127.0.0.5. Yeah it's the same thing. Okay doesn't matter. Okay. So if you look at here, usually what happens the PC you ping someone 8.8. So it may take help from the Google DNS and after that it goes to google.com. Okay. Let's say if you are typing google.com if you are typing uh ping google.com ping google.com. So um there has to be an IP address behind Google. So your PC will go to the DNS and we'll get the IP address for the google.com and we'll go to the google.com.

Okay, this is how it works. But uh with the loop back you actually ping your own your own interface your own nick card. Okay. And uh this is the concept for IPv4. Okay. In IPv6 you also have a loop back. Okay. uh see the range of that one. Um that that is also a loop back over there. Now link local address when you have a DHCP server or you are asking IP address from the DHCP server. Maybe your DHCP server is not working.

Maybe your DHCP server is not there. So you cannot obtain an IP address from the DHCP server. Then your computer automatically assigns assign itself an IP address which is from this range 169.254 169.254 00 to 255 any IP address of this range. Okay. And this IP address is not routable. Okay. That means this IP with this help of with the help of this IP address you cannot go to a different network. Okay. So I'm talking about a particular network.

So if you have a switch and if you have two PCs connected, they're in the same network. I I would expect they're in the same network until and unless they are not in different vans. So he will get an IP address, he'll get an IP address because there is no DHCP. So they will generate an IP address which is also called AIPA and they will be able to ping each other. They will be able to communicate with each other. Okay.

So uh automatically assigns IP address when DHCP fails. It is only for local communication within the same LAN. That is very important. It cannot be used to a different LAN to a different network. Uh useful for troubleshooting. This is this IP address is good for temporary use not for permanent use. Okay. Uh this is only for the local communication. Um so it is better not to use something. Now if you look at here um you see local address.

So you can even without doing anything if you even if you don't set up any IP address no DHCP server you can be you will be able to communicate with each other if they're connected with a switch. If they are connected with the switch now if you look at here let's say this is a PC. So if you look at here if you click here you'll get the static IP address. But I want a DHCP IP address. The same thing I want a DHCP IP address.

But there is no DHCP configured. So if you look at here, he got an IP address 169 254 238 215. He got an IP address 254 2 uh 237.3. You see they got an IP address, right? Who gave this IP address? DHCP failed. It's an IP IP address. So with this one at least like if you want to transfer some files or something between these two computers, it is possible. So if you come here uh let's come here. Let's open the cmd. So if you look at here if you ping uh let's say so if you look at here is already there.

So you see successful the ping is successful. Okay. Now let's say if you want to ping uh so I'm pinging this computer. Let's say if you want to ping 127.5. I made a mistake uh with this one. So you look at here, this is also successful. This is also successful. Why? Because this is a loop back address. You are pinging yourself. Okay. Now u if you ping let's say if you want to see something that is related to unsuccessful.

So look at here now you see request timeout because he don't know how to reach to 8.8.8. He don't know how to reach to 8.8.8. So to recap, what is loop back? Anytime you ping a loop back, you are pinging yourself. Okay? It never leaves your computer. It never leaves your computer. Okay? And it can be any number 127. Number dot any number any number. Okay? It doesn't matter. So if you ping 1.12 127 If you ping 127.1.1 can I ping 127.5?

Yes. Any number you ping at any point in time you will find positive and it is actually pinging you and uh if there is no DHCP these pieces will generate some IP addresses by itself and uh that is only usable inside the network. So if you go outside the network for example there is a router connected and going out of this network you have a switch you have a computer you will never be able to communicate with the link local between these two computers.

Okay. Only inside the network it is possible.