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**Exam Code:**JN0-103

**Exam Name:**Junos, Associate (JNCIA-Junos)

**Certification Provider:**Juniper

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Last Update: May 20, 2024 - Training Course 67 Lectures

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### Networking Fundamentals

**6. Network Devices**

Hello and welcome back. In the last video, we spoke about collision domains, broadcast domains, and VLANs.In this lecture, we will discuss about network devices. First, let's start with what a repeater i.e. repeater is a layer one device which is used to repeat signals. It is not an amplifier, which means it does not improve the strength of the signal, but it only repeats i.e. then have a hub. A hub is a layer one device that operates in half duplex mode. This means you can only send or receive data at any time. A hub has multiple input and output ports, allowing multiple devices to connect at the same time. When data is received on one port, itis forwarded out on all the other ports. Very important. A hub does not have any intelligence of its own, which means it cannot learn Mac addresses. Every time it gets traffic, it just sends the traffic out. All the other ports hubs are not good from security standpoint because everyone receives a copy of the data. We then have a bridge. A bridge is a layer two device that has the capacity to learn Mac addresses. Frame forwarding is software based. When the bridge receives the frame for the first time, it learns the source port and the source Mac address and stores that into the cam table, also known as the switching table. The frame is then forwarded out all ports other than the one on which it was received, because it is not known on which port the destination device is connected. Once you're replies received, the destination port adman address is known and it is added.

**7. Layer 2 loops, Spanning Tree Protocol and Optical Networks**

Alright, welcome back everybody.In the last lecture we spoke about network devices.In this lecture, we're going to talk about layertwo loops standing, three protocol and optical networks.So let's begin.So first up, what is a layer to loop? Let's look at this diagram over here.So we have three switches, switchA, switch B and Switch C.And we have three devices, host A, hostB and host C, which are connected tothe switches A, B and C respectively.Now, if you notice the way the switches areconnected, there is a loop in the connection.Switch A is connected to switch B.Switch B is connected to switch C.And switch C is again connected to switch A.This kind of a topology is known as a layer to loop.Layer two loops are good because they give us back upoptions to fall back upon when working links go down.Layer two loops are bad becausethey cause packets to loop around,forever consuming valuable network resources.Let's try to understand this.So we have three switches connected in a loop andwe have three devices, one device connected to each switch.Now, when host A tries to send a packet to hostB, the packet is first going to reach Switch A.Now Switch A has no idea where host B is connected,so it is going to send the packet out all otherports except the port on which it was received.So one packet goes out towards Switch Band one packet goes out towards Switch C.Now let's talk about the packet that hasbeen received by Switch B, the blue packet.Now Switch B again, it's a new network, soit has no idea where host B is connected.It is going to send the packet out on allother ports except the one on which it was received.So switchb sends the packet towardshost B and towards Switch C.Host B has received the packetwhich was intended for it.But then there's one more packet whichhas been directed towards Switch C.What happens to that? Well, switchc looks at the packet and says, Ihave no idea where host fee is connected.I'm going to forward it out all other portsexcept the one on which I received it.So it forwards one copy towards hostC and one copy towards Switch A.Host C will look at the packet and saythat, hey, this packet is not this time forme, so I'm going to drop the packet.But what about the packet that has reached Switch A? Because Switch A has no idea where Host B is connected, it will send the packet out all other ports except the one on which it was received, so it will send one copy to HostA and one copy to SwitchB. Host A was the source of the packet.The packet has looped around and it has reached host A.Again, host A will look at the packet and say, "Hey, this packet is not designed for me, so I'm going to drop it." But what about the package that is designed for Switch B? Switch B already knows where host B is connectedbecause it already forwarded one packet for host B.So it's going to forward the copy over to Hostb.Now host B has received two copies of the same packet.Now let's talk about the brown packet.The brown packet is the original copy thatswitch A had forwarded towards Switch C.switchc looks at the packet and says hey, Ihave no idea where Hostb is connected so I'mgoing to forward it out all other ports.So it sends one copy over to hostC and one copy over to switchb.Host C looks at it and says hey, it's not me.So I'm going to drop the packet.The other packet that is received by switchb switchbnow already knows where host B is connected soit's going to send it out towards Hostb.Now notice what happened.Host B has received three copies of the same data.This is what we said that layertwo loops can consume valuable network resources.So let's look at one more situationwhere we do not have a loop.So we are back to the same diagram.Let's try to remove the loop.So we can remove the connectivitybetween switch A and Switch B.Or we could also remove the connectivitybetween switch B and Switch C.Or we could also remove itbetween switch A and Switch C.Anything is fine.So let's just do this.Let's just remove the connectivity between switch B andSwitch C and let's say host A is tryingto send a packet over to host B.So host a first forwards the packet to switch a Switch Alooks at the packet and says hey, it's not for me.It's for somebody called Host B.I have no idea where he's connected so I'mgoing to forward it out all other ports.So switch A sends the packet towardsSwitch B and towards Switch C.switchb looks at the packet and says hey, Ihave no idea where host B is connected.Let me just forward out the packet.

**8. IPv4 Address Format & Classes of IPv4 Addresses**

Alright everybody.Welcome back to this lecture on IP versionfour address format and classes of IP addresses.Let's first begin by talkingabout IP version four addresses.An IP version four address address is a 32bit logical address assigned to a network device.Notice the word logical.We spoke about Mac addresses earlier, which was thereal address or the physical address of a device.We also discussed that the Mac address is burned ontothe network device or the chip of the device.An IP address, on the other hand, is alogical address, which means you are free to assignwhatever address you want to the network device.It is written in decimal format.So the numbers that we use every day, starting from 12345and so on, these are all numbers in decimal format.IP addresses are also written in decimalformat with four octaves separated by dots.For example, we have 192, 168, one one.Every number in the IP address canbe represented by eight binary bits.Hence it is known as an octet.So we have these fourdifferent numbers separated by dots.Every number can be represented by eight binary bits.You multiply four with eight, you get a total of 32.That's why we call it as a 32 bit address. We'll.

**9. Decimal to Binary Conversion**

Alright everybody, welcome back to thislecture on decimal to binary conversion.This lecture is very important because unlesswe know how to convert decimal tobinary, we cannot actually start with subnetting.So make sure you have a pen and paper paper with you.We are going to practise different examples ofhow to convert decimal to binary and thenconvert binary back over to decimal.Let's begin.The first example is to convertthe number 16 to binary.Now, before we actually start understanding how todo this, I want to tell you something.If you go to the internet and try to searchfor how to convert decimal to binary, you're actually goingto get so many different methods to do this.I'm going to share with you a methodthat I learned in my school days.It's probably the most easiest method in my opinion.It's really easy and I'm sure you'regoing to love this method as well.So let's start by converting the number 16 to binary.The first thing that you need to do isto draw a vertical line and draw a fewhorizontal lines like what you see right now.Write the number that you need to convert on the top.Now we need to start dividing this number with two.Because we are converting to binary, weneed to divide it by two.So when we divide this with two, the quotientis going to be eight because two multiplied byeight is 16 and the remainder is zero.So write the remainder on the right hand side.Just like this.We need to further divide eight by two.So two multiplied by four is eight and the remainder iszero, which we will write on the right hand side.We will continue doing this, divide four by two, twomultiplied by two is four and the remainder is zero.We can divide this further.So two multiplied by one istwo and the remainder is zero.Now remember, we do not stop here.We have to do this one more time.Now you may say, how do I divide one by two? So we have to do it this way.Two times zero is zero, and the remainder is one.We stop doing this when we have the quotient of zero and the remainder of one.We have it right now, so we're going to stop the process here.Now to get the answer, we need to write these numbers in reverse order.We're from bottom to top.So 16 in decimal is equal to 10.Zero in binary.Isn't that easy, guys? Let's try to do one more example.Let's convert the number 31 to binary.We first start with a vertical lineand a set of horizontal lines.We write the number on the top right and we startdividing by the number two to multiplied by 15 gives youthe answer of 30 and it gives you a remainder ofone, which we will write on the right hand side.Divide it one more time.Two multiplied by seven is 14 andyou have the remainder of one.One more time.Two multiplied by three is six andyou get the remainder of one.Two multiplied by one is two and the remainder is one.Now remember what we said.We stop only when the quotient iszero and the remainder is one.The quotient is not yet zero.So we need to do this one more time.

So two multiplied by zero iszero and the remainder is one.So now we have the quotient ofzero and the remainder of one.The answer is in the reverse order from bottom to top.So 31 in decimal is equal to one.One one in binary.So you guys are getting better.Let's try one more example.Let's convert 52 to binary.We start with a vertical lineand a set of horizontal lines.We write the number on the topright and we start dividing by two.Two multiplied by 26 is 52 and the remainder iszero, which we write on the right hand side.One more time.Two multiplied by 13 is 26 and the remainder is zero.Two multiplied by six is twelveand the remainder is one.Let's do it again.Two multiplied by three is sixand the remainder is zero.Two multiplied by one is two and the remainder is one.Now we do not stop here.We have to get the quotient as zero.So we need to do it one more time.So two times zero is zero and the remainder is one.We now have the quotient aszero and the remainder as one.The answer is in the reverse order.So 52 in decimal equals 110100 in binary.Alright, now let's do the other one.We now understand how toconvert binary back to decimal.So let's do it.Let's try to convert this number which is 10 101.It is in binary format.We need to convert it over to decimal.It's very simple.First of all, we'll write thenumber two under every digit.So under every one and every zero that yousee over there will write the number two.Next we need to assign the powers of two.We'll start with the rightmost one.The rightmost one has the power of zero.The one on the left has the power of one.The next one has the power oftwo, three, four and so on.So we need to start with two power, zero onthe rightmost end, and then increment the power one byone as we move to the left hand side.Next, we need to add these powers of two.However, we do not add those powersof two corresponding to the zeros.So if you notice here we have two zeros.We have 10 over here and another zero over here.We are not going to add the powersof two corresponding to these two zeros.We'll add the remaining powers of two.So we're going to add two power fourplus two power two plus two power one.Two power four is 16.Two power two is four and two power zero is one.We add these values, we get the number 21.

So 10101 in binary equals 21 in decimal.Isn't that easy, guys? I'm sure you guys are having fun.Let's do one more example.Let's try to convert 10 zero into decimal.What's the first step? The first step is to write the number two corresponding to every digit in there. So we've written five twos. What's the next step? The next step is to assign the powers of two.We start with two powers zero on the rightside and then we move to the left handside by incrementing the power of two.So we have two power zero, two power one, twopower two, two power three and two power four.The next step is to add these powers.However, we only add those powers of two correspondingto the ones if we have zero in there.We do not add the powerof two corresponding to that zero.Here we only have a single one,so that's the only one that matters.Two power four is 16.Rest of them are zeros.10 zero in binary equals 16 in decimal.I'm sure you guys are loving thisand you're also becoming experts in this.Let's do one more example.Okay, so let's try to convert 1101 into decimal.That's a big one, but it's very easy.The steps are the same.What's the first step, guys? The first step is to put the number two against each of these digits. So I've placed all the twos. The next step is to assign the powers of two. We start with two powers zero on the right side and increment the power of two as we move to the left side. So we start with two powers zero on the right side. And as we move along, we reach two powers seven on the leftmost side. What's the next step?

The next step is to add the powers of two.But we only add the powers of two that correspond to the ones.If there is a zero in there, we will not add the power of two that corresponds to that zero.So we have 10, the rest are all fine.So two power seven is 128.Two power six is 64.Two power five is 32.Two power four is 16.Two power three is eight.Two power two is four.And finally, two power zero is Are we in school? That's okay guys, let's just think we're in school. We're trying to learn something new. So what's the homework for you? The homework for you is to convert 85, 43, and 101 into binary. Also, you need to convert these binary digits into decimal digits.So make a note of these and make sure you practise them because we are going to discuss the answers in the next lecture.So what are my important tips? Just remember and practise how to convert decimal to binary. Also, remember and practise how to convert binary to decimal.In the next lecture, we're going to start understanding what a subnet mask is. We'll understand how a subnet mask is helpful in separating the host part and the network part in an IP address.And most importantly, we'll understand why we even need subnetting. I promise you that's going to be the last lecture before we get our hands dirty with subnetting. That's all for this lecture, guys. I'm sure you loved this lecture. Before I catch you for the next lecture, I want you to make sure you've done your homework, alright? So if you have any questions, please let me know. Otherwise, I'd like to thank you for watching, and I'll catch you in the next lecture.

**10. Subnet Mask & It's Importance**

Alright everybody, welcome back. In this lecture, we'll talk about what a subnet mask is and understand the importance of subnet masks. Before we start, let's discuss the answers to the homework from the previous lecture. So here are the answers. For decimal to binary, the first number was 85. Now 85 is 83 in binary, and one is 110-0101 in binary. From binary to decimal, 1101 in binary is 29 in decimal. 100-1001 in binary is 73 in decimal. And the final 110-0100 in binary equals 1 4 5 in decimal. I hope you all got them right. If you have not got them correct, please look back at the last lecture and try to find out what may have gone wrong. So now let's begin with subnet masking and its importance. So first up, what is the subnet mask? Every IP address has two parts. One part is known as the network portion, and the other part is the host portion. To understand this, think about your home address. You have a street name in which you live, and then you have your house number. In the exact same way, you have the IP address, which has the network portion, and it also has the host portion. Every IP address is always accompanied by a subnet mask.

A subnet mask is also a 32-bit number that helps you separate the network portion from the host portion. There are two ways to denote a subnet mask. For example, you could write it as 100 eight, or you could also write it as ten 00:25 50.Both of them actually represent the same IP address. When the subnet mask is converted to binary, the ones in the subnet mask denote the network portion and the zeros denote the host portion. For example, let's look at TEN 002550. So in decimal, it is 2550. When we convert it into binary, you have the first octet, which is all ones, and then you have the last three octets, which are all zeros. So the first eight bits are made up of ones, and that is the network portion. The remaining three octaves are all zeros. That is your host portion. Since the first eight bits are made up of ones, you have the notation "ten." The number after the slash indicates the number of network bits. Let's look at one more example. We have 192-168-1024, or one nine of 2162. So in decimal, it is written as 255, 255, 2550. When you convert it over to binary, you have the first three octets made up of all ones, and you have the last octet, which is made up of zeros. So the first 24 bits, which are made up of ones, indicate the network portion. Hence, you have the notation 24. The last eight bits are made up of zeroes. That is your host portion. So you get the idea how the subnet mask plays an important role in dividing the IP address into two parts. In the host portion and the network portion, we discussed classes of IP address.

In the last lecture, we said there are five classes of IP address: class ABC, C, D, and E. We also discussed that class D is reserved for multicast purposes and class E is reserved for experimental purposes. So you're left with only three classes: class A, B, and C. Now, each of these classes has a default subnet mask associated with it. Class A has a default subnetmask of eight. For example, 122 eight. Class B has a default subnet mask of slash 16. For example, 172 is dot zero slash 16. And class C has a default subnet mask of 24. For example, 192, 168, ten, 00:24. All right. Now let's talk about the total number of hosts. So the total number of possible host IP addresses on a network equals two powers of H, where H represents the number of host bits or zeros in a subnet mask. Very important. Please remember, the first address is the network address, and the last address in the range is the broadcast address of the network. So, subtract two IP addresses from the total number of hostaddresses you have, which is two powerH. the first one, which is the network address, and the last one, which is the broadcast address. So the total number of usable IP addresses equals two powers of H minus two. This is a highly possible examination question. They will give you an IP address with a separate mask, and they will ask you to find out the total number of usable IP addresses. So, first up, you need to find out how many network bits you have and how many host bits you have. The total number of possible host IP addresses equals two powers, the number of host bits. You then have to subtract two addresses. The first one is the network address, and the last one is the broadcast address. What you're left with is the total number of usable host IP addresses. Let's look at an example. So, for example, we have this IP address: 100, zero, slash eight. The first eight bits represent the network portion; the remaining 24 bits Because the total number of bits is 32, you subtract eight from 32, so you're left with 24 bits for the host portion. The total number of possible IP addresses would be two powers of 24. The total number of usable IP addresses would be two powers lower, or 24 minus two. Let's now understand why we need subnetting. Consider the public IP address. 110, 0:8, and 8: Here the first eight bits are occupied by network bits. This leaves 24 bits as the host bits. The total number of usable host addresses would be two powers of 24 minus two, which is 167-7214. Now, that's a lot of IP addresses on a single network. Such a large network would be extremely difficult to manage. Plus, you have the problem of broadcast traffic. We discussed broadcast when we were talking about layers. Two concepts.

If you have such a large network, you have so many devices connected on this large network. And every time a device sends a broadcast, imagine the number of devices the broadcast is going to reach. It is going to be extremely difficult to manage a network of this size. So subnetting can help you break this large network into smaller, much more manageable networks. Let's look at one more example. So consider a point-to-point network between two routers. You have Router A on the left hand side, and you have Router B on the right hand side, and you need to connect them up. How many IP addresses would we need? We only need two IP addresses, one for each router. We just discussed the default subnet masks for class A, B, and C, and the smallest subnet mask is 24. Let's say we assign a 24-bit IP address to this link between router A and router B. So in the smallest subnet mask, which is slash24, you have the first 24 bits, which are occupied by the network bits, and you have eight bits that are left for host bits. So the total number of usable IP addresses would be two power eight minus two, which is 256, minus two, which is 254. So imagine that we have a point-to-point link between two routers, which only needs two IP addresses. But the smallest subnet mask possible gives us a total of 254 IP addresses. Once I use two IP addresses, I am still left with 252 usable IP addresses. That is such a waste of IP addresses. Now, imagine if we had a slash 30-subnet mask. Now, I know when you look at this, you might feel confused because we've not discussed subnetting yet. We've only discussed subnet masks such as 816 and 24.

So 30 looks new to all of us. Let's just assume that we have a 30-bit IP address. So out of the 32 bits, the first 30 bits are taken away for network bids, and we are only left with two bits for host bids. The total number of usable IP addresses would be two power two minus two, which is two. I have exactly the number of IP addresses that I need. To connect both these routers, I only need two IP addresses. I have two IP addresses available. That is such an efficient utilisation of the IP address space. That is one of the biggest advantages of subnetting. It allows you to efficiently use the public IP address space. So what are my important tips? Remember, every IP address must be accompanied with a subnet mask. Without a subnet mask, an IP address is of no use. A subnet mask is also a 32-bit address, just as you have the IP address, and it allows you to separate the network portion from the host portion. Subnet mask has two possible notations. For example, you have 192-168-1024. Or you could also write it as 1981-6810-2552-5525 50.The total number of possible host addresses equals two powers of H, where H represents the total number of host bits in the subnet mask or the total number of zeros. The total number of usable IP addresses would be two plus h minus two because the first IP address on the network is the network address and the last IP address on the network is the broadcast IP address. In the next video, we are going to start with examples of subnetting. We're going to get our hands dirty with lots of different examples of subnetting. Are you guys excited? I'm sure you are. When you come to the next lecture, make sure you have a pen and paper handy because we are going to practise different examples of subnetting. That's it for this lecture, guys. If you have any questions, please let me know. Otherwise, I'd like to thank you for watching, and I'll catch you in the next lecture.

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