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In this section we gonna look at IP subnetting.

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I’d like to teach you multiple methods for subnetting.

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Subnetting is one of those subject that a lot of Network Engineers struggle with.

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But in this section I hope I'll be able to simplify for you

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and give you various methods.

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So that any subnetting questions that you might encounter

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can be completed quickly and easily.

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There are 2 main parts in this section:

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in the 1st part I'm gonna show you 2 methods to quickly and easily answer

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questions like, what subnet is this host on or what is a last host in this subnet?

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So I’m gonna teach you the binary method and then

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I’m gonna teach you what I call the quick  method.

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I suggest that you look at both and then choose one

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and get good at that specific  method.

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I think binary is very important to understand.

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But as the quick method name indicates it’s very quick and easy

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to answer subnetting questions  using the quick method.

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In the 2nd part I’d like to show you how to create multiple subnets

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when given a specific network or subnet, so you will be able to answer questions

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like subnet this network into  at least 10 subnets or subnet this network

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into subnets each having 10 hosts on them.

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So in part 1 you'll be given an IP address and then you gonna need to learn

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how to work out the  subnet address, 1st host address, last host address

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and the broadcast address and we'll gonna use 2 methods: the binary method

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and the quick method, and as I mentioned I suggest that you choose 1  of the

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2 methods, look at both, decide which one you prefer

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and then learn and get good at that specific method.

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So here's the typical example:

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You’ve been  given a host with an address of  192.168.10.18 for the /24 mask

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and you’ll  need to be able to answer the  following questions:

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what IP address with router 1 be configured with, this router being router 1

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if it is to use  the first IP address in the same  subnet as PC 1?

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Which is the PC here? The second question could be:

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What broadcast address is used by PC 1?

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or thirdly what IP address would router 1 be  configured with if it is to use

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the last IP address in the same  subnet as PC1?

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So the first question was, what IP address  will the router use if it is

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configured with the first IP address?

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And the third one is: what IP address would it be configured with if it were

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configured with the last IP address in the subnet?

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And fourthly, a question could be what subnet is PC 1 part of?

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so you should  within 30 to 60 seconds be able  to answer those question

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and I’m  gonna show you how to do that now.

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So the first method is to use binary to work out the answers.

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I personally prefer binary because it's used all over the place in networking.

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And I want you get your head around how binary works.

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It will save you a lot of time in the long run.

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Please refer to the binary section of this course

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if you are not sure about the basics of binary.

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we are  going to assume knowledge of  binary now

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we're just gonna use binary to work out the answer to those questions.

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In the binary method, you need to remember 4  rules

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to work out the network with subnet address

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you fill the  host portion of the address with  binary 0's

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So you work out which part is network and which part is host

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and you fill the host portion with binary 0's.

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To work out the broadcast address,

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you fill the host portion of the address with binary 1's.

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To work out the first host in the subnet, you fill the host portion of the

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address with binary 0's except for the very last bit which is set to a binary 1.

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And to work out the last host in the subnet, you fill the host portion of the

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address with binary 1's except for the last bit which is set to binary 0.

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If you remember those 4 rules, you will be able to work out the answer to any

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of those questions very  quickly.

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So let’s start the basic example.

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We have a PC with an IP address 192.168.1.18/24 or it could be written as

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192.168.1.18 with the mask of 255.255.255.0

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As you learnt in the IP addressing section, these 2 are equivalent subnet mask.

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It's very similar to saying tomato or tomato or router or router.

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It means the same thing it's just a different way of displaying the subnet mask.

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Now hopefully you remember that 24 means 24 bits in the subnet mask

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in other word 8 bits in the first octet, 8 bits in the second octet

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and the 8 bits in the third octet. That means the first 3 octets are network.

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So 192.168.1 is the network portion of the address and this last octet is the

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host  portion and that’s how I’ve written it here.

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The first 3 octets are displayed in black representing the network portion and

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the last octet is shown in red which represents the host portion of the address.

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There’s no point converting the network  portion into binary.

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So I’ve just left as it is, so the subnet is equal to 192.168.1.

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and if you remember back to our rules to get the subnet,

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you fill the host portion of the address with binary 0's

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So this is the  single octet, so 8 binary 0's need to fill  that host portion.

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If you convert 8 binary 0's back into decimal you'll get the value 0.

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So the subnet is 192.168.1.0 for this specific IP address.

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To get the first host in the subnet, you fill the host portion of the address

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with binary 0's except for the last bit which make binary 1.

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So an octet once again is 8 bits.

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So there are 7 binary 0's followed by single binary 1.

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Converting that back to decimal you get 1,

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so the first host in this subnet is 192.168.1.1

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To work out the last host, fill the host portion of the address with binary 1's

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except for the last bit you set to binary 0

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That's 7 binary 1's followed by binary 0, if you convert that back to decimal

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you'll get the value 254.

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So the very last host in this subnet is 192.168.1.254

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To get the broadcast address, fill the host portion of the address

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with binary 1's, so single octet fill with binary 1's is equal to 255 in decimal

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So there you have it. We can work out the subnet, the first host, the last host

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and the broadcast address very quickly and easily using the binary method.

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So if you were ask to configure the 1st IP address in the same subnet as PC 1

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on the routers interface you would know that you need to configure the IP address

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192.168.1.1/24 or 192.168.1.1  255.255.255.0

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You could be expected to type the commands unto the router to configure it with

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the relevant IP address, later in this course you'll learn the commands to do that

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But for now, realize that you might need to work out first, last, broadcast and

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subnet addresses When giving a specific IP address.

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This is our first example using the binary method

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I’d like to show you more complicated example now.

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So in this example you've been given an IP address 172.16.35.123/20

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or in other words 255.255.240.0

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This is a little bit more complicated because the mask

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does not fall on the octet boundary,

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in other words, it's not a simple /8 or /16 or /24 subnet.

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The network and host portion lies somewhere in the third octet.

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So just to reiterate, /20 means that 20 bits of the 32 bit IP address

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which are used for networks or subnet portion and the remaining 12 bits are used

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as the host portion of the address.

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So somewhere in the third octet, the network host portion changes.

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So once again our IP address is 172.16.35.123/20

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Now the first octet is 8 bits in size , the second octet is 8 bits in size.

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So adding those together you get 16 bits.

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The third octet is 8 bits in size, so adding those 3 together gives you 24 bits.

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So /20 in the subnet mask means that the split between subnet

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and host occurs somewhere in the  third octet.

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So once again the first octet is 8 bits, second octet is 8 bits thus

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20 bits puts the split somewhere in the third octet.

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So what you need to do is convert the third and fourth octet

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into binary because they both have host bits.

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There’s no need to convert the first 2 octets into binary

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because no host bits are found in the first 2 octets.

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So 00100011 is the binary equivalent of decimal 35, 0 followed by 4 binary 1's

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followed by 0 followed by 2 binary 1's is the binary equivalent of decimal a 123.

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So all we’ve done is convert 35 and 123 into their binary equivalents.

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Please see the binary section of this course if you are not

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sure how to convert decimal into binary and back again.

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20 bits of subnet mask put us in this position because we have 8 bits

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plus 8 bits in the first 2 octets which give us a total

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of 16 binary bits plus 1 is 17 plus 1 is 18 plus 1 is 19 plus 1 is 20

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So we can draw a line at this point to split the network and host portion

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To the left of this line, the binary bits indicate network or subnet,

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to the right of this line the binary bits indicates host.

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So there are split between network and host portions in this address at this line.

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So to work out the answers you need to remember the 4 rules.

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The first rule states that, to work out the network or subnet address,

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you need to fill the host portion of the address with binary 0's.

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So to the right of the line we need to fill

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this entire portion of the address with binary 0's.

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So once again 20 bits of subnet mask put us in this position

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and we fill everything to the right with binary 0's.

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So we end up having 172.16 the first 4 bits of the third octet

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remain the same and all remaining bits are set to binary 0.

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When you convert that back to decimal, please note firstly that these 4 bits

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in red plus the following 4 bits in green are part of the third octet.

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So 0010 followed by 0000 when converted from binary

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to decimal gives us a value of 32. The fourth octet is filled with 0's.

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So converting an octet filled with binary 0's to decimal give us a value of 0.

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So the subnet address is 172.16.32.0

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Now to work out the first host address, fill the host portion of the address

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with binary 0's except for the last bit which is set to binary 1.

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So here you go, the first host is equal to 172.16.0010

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remember, we don’t touch this portion of the address,  we only change the host

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portion of the address, so we fill the host portion with binary 0's

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except for the last bit which we set to binary 1.

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Once again the third octet consist of 4 binary bits which are part of the network

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or subnet and 4 binary bits which a part of the host portion.

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Those 4 red binary bits together with those 4 green binary bits

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give us a decimal value of 32.

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The last octet is 7 binary 0's followed by  single binary 1

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which is equal to decimal 1, so the first host address is 172.16.32.1

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To work out the last host address, fill the host portion of the address

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with binary 1's except for the last bit which is set to binary 0.

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So the last host is equal to 172.16.0010 followed 4 binary 1's, in the third octet

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followed by 7 binary 1's in the fourth octet and then lastly a binary 0.

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So the third octet consist of this 4 red bits and this 4 green bits.

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So if you convert 0010 followed by 4 1's into decimal

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you'll get the decimal equivalent which is 47, 7 binary 1's

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followed by binary 0, gives you the equivalent decimal value of 254.

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So the last host in this subnet is 172.16.47.254

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To work out the broadcast address, fill the host portion

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of the address with binary 1's.

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So the broadcast address is equal to 172.16.0010

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Once again, the network and subnet portion are left as is.

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And then you fill the host portion with binary 1's.

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So the third octet is equal to 0010 and 4 binary 1's

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which once again gives you the decimal equivalent of 47.

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8 binary 1's in octet gives you the decimal value 255.

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So the broadcast address would be 172.16.47.255

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So in summary, an address of 172.16.35.123 with the subnet mask of /20 resides

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on subnet 172.16.32.0 and has a broadcast address of 172.16.47.255

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the first host in the same subnet is 172.16.32.1

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and the last host in the same subnet is 172.16.47.254

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This was a slightly more complicated example, but I’m hoping that by now

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you have learned the process to work out subnet first host, last host and

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broadcast address when given an IP address and its subnet mask.