Cisco CCNA ICND2 640-816

Advanced TCP/IP: IP Subnetting, Part 3

by Jeremy Cioara

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Video Title Duration

Review: Rebuilding the Small Office Network, Part 1

Review: Rebuilding the Small Office Network, Part 2

Review: Rebuilding the Small Office Network, Part 3

Switch VLANs: Understanding VLANs

Switch VLANs: Understanding Trunks and VTP

Switch VLANs: Configuring VLANs and VTP, Part 1

Switch VLANs: Configuring VLANs and VTP, Part 2

Switch STP: Understanding the Spanning-Tree Protocol

Switch STP: Configuring Basic STP

Switch STP: Enhancements to STP

General Switching: Troubleshooting and Security Best Practices

Subnetting: Understanding VLSM

Routing Protocols: Distance Vector vs. Link State

Routing Protocols: OSPF Concepts

Routing Protocols: OSPF Configuration and Troubleshooting

Routing Protocols: EIGRP Concepts and Configuration

Access-Lists: The Rules of the ACL

Access-Lists: Configuring ACLs

Access-Lists: Configuring ACLs, Part 2

NAT: Understanding the Three Styles of NAT

NAT: Command-line NAT Configuration

WAN Connections: Concepts of VPN Technology

WAN Connections: Implementing PPP Authentication

WAN Connections: Understanding Frame Relay

WAN Connections: Configuring Frame Relay

IPv6: Understanding Basic Concepts and Addressing

IPv6: Configuring, Routing, and Interoperating

Certification: Some Last Words for Test Takers

Advanced TCP/IP: Working with Binary

Advanced TCP/IP: IP Subnetting, Part 1

Advanced TCP/IP: IP Subnetting, Part 2

Advanced TCP/IP: IP Subnetting, Part 3

00:00:00 - Alright, it's the final piece IP subnetting part three.
00:00:05 - And thankfully this one's not that difficult at all. It's considered
00:00:09 - reverse engineering subnets. Up to this point we've talked
00:00:13 - about how to subnet based on the number of networks. Based on
00:00:16 - the number of hosts and now we're going to look at working
00:00:19 - it backwards reverse engineering subnets. Now as I mentioned
00:00:22 - in the previous video this is the most common style of subnetting
00:00:27 - you're going to find in both the real world and in the certification
00:00:31 - exam. It's essentially a type of subnetting that says here's
00:00:35 - the answer, now work backwards. We'll finally wrap up the whole
00:00:39 - subnetting section by talking about the great exception to
00:00:42 - the rule and I'll explain that once we get there. So let's get
00:00:46 - going with the reverse engineering.
00:00:49 - Here is a perfect example of reverse engineering a subnet problem.
00:00:53 - We have a host or a router or a switch on our network
00:00:58 - and we look at it and get this IP address information.
00:01:01 - We see it's IP address and we see it's subnet mask and we're faced
00:01:05 - with the question of is this IP address even valid? What network
00:01:09 - range does it belong to? Or you know, what network is this
00:01:14 - part of? Is it on the same network as the other hosts that are
00:01:17 - plugged into the same switch and all that. All of these questions are
00:01:20 - honing in on the reversed engineering. Meaning we see the IP
00:01:23 - address and we see the subnet mask and we want to know what
00:01:27 - network range that IP address belongs to. So when you're faced
00:01:31 - with a problem like this and you're trying to figure it out
00:01:35 - we need to work backwards and in my mind it's a little easier
00:01:38 - than the ones we've been doing up till now. The number of networks
00:01:41 - and the number of hosts. We don't have to do any of the conversion to
00:01:44 - binary of those numbers and reserving bits and so on because
00:01:47 - we have the answer. This is the answer. Somebody figured out
00:01:52 - the subnet mask for this host way back when and in that subnet
00:01:56 - mask lies the key.
00:01:58 - We want to know what increment was used when we figured this
00:02:03 - all out. Meaning this IP address right here belongs to a range
00:02:07 - and the only one way that we can know the network range is to
00:02:10 - reverse engineer this. Work backwards and figure out what was
00:02:13 - the increment. The way we do that is take the lowest subnet mask
00:02:18 - AC Tech or the non 255 subnet mask and break it in to
00:02:22 - binary. So 224 in binary if we were to lay out our binary chart
00:02:26 - 128 64 32 16 8 4 2 1
00:02:32 - 224 is actually one one one zero zero
00:02:37 - zero zero zero.
00:02:39 - Now you can subtract that out and work with it, but after you see
00:02:43 - the subnet mask values for you know, so long you're going to start going, oh, that's
00:02:47 - three ones. Now what increment was used with that subnet mask? Well,
00:02:53 - the lowest network bit
00:02:55 - that one so it is 32. So 32 was the increment
00:03:00 - that they used when they found these network ranges so we'll
00:03:03 - start where they had to start when when this subnetting was reversed
00:03:07 - engineering and when I say they I mean the network designers
00:03:10 - or whoever planned this scheme. So I put
00:03:13 - and just start adding 32. 32, 34,
00:03:18 - 96, 128.
00:03:23 - And we could keep going but we've passed the IP address so
00:03:28 - we can work backwards and find out, oh, this is the network
00:03:32 - range that they came from. 192.168.1
00:03:37 - through, oops, 127. You can
00:03:42 - still see it. That's the network range that we're working
00:03:45 - through. Is this IP address valid? Absolutely not. At least not
00:03:50 - to be assigned to a host because that is the last IP address
00:03:53 - in the range which makes it the broadcast address which
00:03:57 - the only way that you'd be able to figure that out is by doing
00:04:00 - reversed engineering and seeing the error of assigning that
00:04:03 - IP address to the host.
00:04:06 - Let's move on to scenario number two. Once again we have an
00:04:09 - IP address assigned to a pc 172 16 68 65
00:04:13 - We've got the subnet mask listed below and a default gateway is
00:04:18 - assigned, 172 16 68 62. IP address down
00:04:22 - here of the default gateway is 172 16 68 62
00:04:26 - looks good that's going to be the gateway the computer uses
00:04:29 - as it goes out. And we see the subnet mask of the default gateway
00:04:33 - is the same. Everything looks good and maybe down here
00:04:36 - is the internet. Now this scenario might be a typical test
00:04:40 - question. It might be a scenario in your network environment.
00:04:44 - Either way, the only way you're going to know if everything
00:04:48 - is kosher on here is by doing the reverse engineering. So once
00:04:52 - again we'll look at the subnet mask because therein lies the
00:04:55 - key. We see 255 255 255 240.
00:05:00 - If we were to break 240 into binary
00:05:04 - I should just have a template that I paste up here on of my binary numbers.
00:05:09 - But get used to writing them. 240 in binary is one one one one zero
00:05:14 - zero zero zero. So
00:05:18 - there's our binary value, which means our increment at some point
00:05:22 - must have been lowest binary value 16.
00:05:27 - So when the designers of the network put this together they
00:05:30 - decided 16 was their increment. Well, let's figure out and see
00:05:33 - if we can find what's wrong with this picture or if everything
00:05:37 - is right. We've got 172 16 68 0 where we'll
00:05:42 - begin, and just start adding 16. 16, 32, 48
00:05:47 - 64,
00:05:51 - 80, if we add 16 and we're good, we've passed both those numbers.
00:05:55 - Now, before we look back at this scenario
00:05:59 - you may wonder well, isn't this a class b network to start off
00:06:02 - with? Shouldn't you have started from 172 16 0 0.
00:06:05 - zero. Well I guess you could have, because whoever subnetted
00:06:10 - this out initially probably started there. But the catch is
00:06:14 - that when you're incrementing by these values you're always
00:06:17 - going to loop back around. Meaning if I started from one seventy
00:06:19 - 172 16 0 0 and started adding 16 and that for AC tech,
00:06:24 - I'd go all the way up to 256 and then go plink,
00:06:27 -
00:06:30 - dot one dot zero. Over here. And then we add up of all the values
00:06:35 - to 256 and then go to 2.0. So the point is that you would
00:06:38 - eventually reach 68.0 by counting by 16
00:06:42 - you just take a lot more paperwork to do it. So instead we can
00:06:45 - just start from the zero in the last AC tech where this this
00:06:50 - network range would have started. And if you look
00:06:53 - we passed both of these IP addresses. 65, well, that one
00:06:57 - shows up right here and 62, that one shows up
00:07:03 - right here. So wait a sec, we have a major issue. This pc is
00:07:09 - assigned an IP address in a different network than its default
00:07:13 - gateway. It belongs to this network range
00:07:17 - through 79
00:07:21 - that's where the pc is at, and the router is at the network
00:07:24 - range above it. That's a big problem. Pc's even if they're plugged
00:07:28 - in to the same switch like this one is to the router. If that device is
00:07:32 - on a different network it will not be able to reach it. It's
00:07:35 - it's not going to know that that device is reachable because
00:07:38 - when it compares its subnet mask
00:07:41 - and it ties this all together the pc goes wait a sec, my default gateway
00:07:45 - is not even on my network, it must be somewhere else. Meaning
00:07:49 - I would need a default gateway to reach my default gateway. If this
00:07:52 - this is how it would all work out. So this would indicate a communication
00:07:56 - problem. This would be the breakdown. And now you can see why
00:07:59 - this is the most common style of subnetting not only on the
00:08:02 - exam because it's very easy to embed something like that
00:08:05 - into a much bigger question. You know, a scenario might say
00:08:08 - oh you know PC A here can't access the internet. And there will be
00:08:11 - a bunch of other scenarios in here of routing tables and protocols
00:08:15 - and blah blah blah, to try and throw you off. But really they're just not on
00:08:18 - the same network at all. They're not in the same range. Same thing
00:08:22 - in the real world. You'll need to know if devices are not on
00:08:25 - them same network because that's where you need a router between
00:08:28 - them in order for them to access each other. The way we would
00:08:32 - fix this is maybe over here the router's connected to another
00:08:36 - ethernet network with some PCs. And over here maybe this
00:08:40 - IP address is assigned the dot 62 but this interface would need
00:08:45 - to have an IP address from the same network range that this
00:08:49 - host belongs to. Some, you know, maybe
00:08:53 - dot 49 would be valid for this one. So host A can reach it
00:08:57 - and get through that router to another network. That's what routers
00:09:01 - do is bridge networks and connect them together.
00:09:05 - So at this point we've seen all three styles of something that
00:09:08 - you can have. Figure it out based on the number of networks. Based
00:09:12 - on the number of hosts, and reverse engineering.
00:09:15 - I want to mention one more document that I've created for the
00:09:18 - series and that is also available on I realize
00:09:23 - that, you know, many of you may have been going through the examples
00:09:27 - at the end of every chapter and a lot of times it is good
00:09:30 - to just have some practical examples sitting in front of you
00:09:34 - that you can work off of without needing to go through the
00:09:37 - whole video to to relearn the whole process. Let me show you
00:09:40 - what I mean and this will make a little more sense. I've created
00:09:42 - a document it's in pdf format.
00:09:46 - Let me see if I can shrink it down a little bit more there we go. Pdf format right
00:09:49 - on the website. It is subnetting examples. What I have is in a
00:09:53 - text document there are three types of examples that I'll show, one
00:09:57 - based on the number of networks one based on the number of
00:09:59 - hosts, or host per network or clients. And then also an example
00:10:04 - of how to reverse engineer it. So what I've got is you can see subnetting
00:10:08 - style one, subnetting based on the number of networks. And I tried
00:10:11 - to, as well as I could, walk through my thought process on paper
00:10:16 - when i'm working through. These kind of like a when I was talking
00:10:19 - through them in the videos that's that's my thoughts. And this
00:10:22 - is kind of me writing my own thoughts. You know, we've got a
00:10:25 - service provider given this you must break it into 20 subnets,
00:10:28 - so step one determine subnets and convert to binary. So there
00:10:31 - it is. Second step reserve the bits in the mask and find the increment
00:10:35 - and we can see the binary tells you how many bits your regional
00:10:38 - subnet mask is this so that's our original subnet mask in all binary
00:10:42 - we must convert five of them to network bits. So you can see what I
00:10:45 - did was try and write what my thoughts were in paper so you
00:10:49 - can always print this out and refer to it in any time. I did one
00:10:52 - subnetting style one is an example based on the number of networks
00:10:56 - with a class C range. The second one is based on the number
00:11:02 - of networks or subnets with the class B range because I
00:11:05 - wanted to give you a feel for each style. Class A is you know
00:11:08 - just like class B so I didn't really include one of those. We have style
00:11:12 - number two right here based on a number of clients. And then style
00:11:15 - number or sorry example number two and is based on the number
00:11:19 - of clients with a class B address. So I did a class C and a class B for
00:11:22 - each one of them. And then finally I have subnetting style three which
00:11:27 - is given an IP address and subnet mask find the network
00:11:31 - range. That's the reverse engineering that we just worked through
00:11:34 - and that you've got a full example of how that works as well.
00:11:37 - So I really encourage you to download that, Print it out, review
00:11:40 - ita couple times. It'll kind of solidify everything that we've been
00:11:43 - talking about in one place. There is one more thing that is
00:11:46 - in this document and that's what I want to talk about right
00:11:48 - now. It is what I would call the great exception.
00:11:54 - At this point you've probably seen enough examples to get
00:11:58 - a feel for how this works. Meaning, how the process of subnetting
00:12:02 - works. When I say oh we've got 30 hosts per network and
00:12:08 - I say our first step is to break that in to binary, the
00:12:11 - reason that i'm doing that and I'm converting 30 to binary
00:12:14 - is because I want to know just how many hosts per network I'm
00:12:19 - going to need. I think that's the number for 30. I was just doing
00:12:22 - that while I'm talking and if that's right I must be really good.
00:12:25 - But it's probably wrong so my my thought is that you know
00:12:29 - the reason I have you break that into binary is because I need
00:12:32 - to know how many bits it takes to get the number 30 because
00:12:35 - that will tell us how many bits we need to carve out of our subnet
00:12:38 - mask or save, save the host in order to get an accurate answer.
00:12:44 - Now the great exception is kind of related to this whole
00:12:48 - process and how binary works. Because binary begins counting
00:12:52 - from zero these networks values may throw off your calculations
00:12:58 - for the number of networks and these values may throw off your calculations
00:13:03 - for the number of hosts. Let me take one of them. Take, let's say
00:13:07 - you've been given 200 here let me just clear a little
00:13:11 - room down here. You've been given the IP address two hundred
00:13:18 - And the question says, well
00:13:23 - break that into let's do 16. Break that in to 16
00:13:28 - networks. Well when you figure that out in binary we would say
00:13:32 - ok there's no one twenty eights, no sixty fours, no thirty
00:13:35 - Oh, one 16 and everything else is zero so we'd say, okay
00:13:40 - in that case it would take five bits to get 16 networks.
00:13:45 - But we can actually get it with four. Four bits are what's
00:13:52 - only required to get 16 networks and the reason that exception
00:13:55 - pops in there is because we started counting from zero. Meaning,
00:13:59 - what's the biggest number you can get with 15 or oops,
00:14:02 - gave you the answer there. What's the biggest number you can
00:14:05 - get with four bits? Well, if we count them up, let me just write
00:14:08 - the binary values above. 128, 64, 32, 16
00:14:12 - 8, 4, 2 and 1. The biggest number that we can get if
00:14:16 - we added up 8 4 2 and 1 that would be 12, 14,
00:14:21 - 15 right?
00:14:23 - But, remember when we start counting in binary we start with
00:14:27 - zero through 15.
00:14:30 - So technically we can get 16 values or 16 numbers
00:14:35 - with four bits. Zero through 15 and that's why any time
00:14:39 - you land on one of these significant binary boundaries, like somebody
00:14:43 - says well give me 64 networks and you put a one right
00:14:46 - there. Well, you can actually get 64 with six bits rather
00:14:50 - than the seven like it looks like it it requires. The same
00:14:53 - thing happens in the host values.
00:14:56 - The following a host values might throw off your calculations.
00:15:00 - Because let's say they said we want networks of seven
00:15:04 - hosts per networks. Well, you figure that out and you go okay, no
00:15:07 - one twenty eights, no sixty fours, thirty twos, sixteen eight,
00:15:12 - oh, four that leaves us with three, so two, one. That is seven in binary. Well
00:15:18 - when we're working with hosts it says well we need you know
00:15:22 - three different value, you know, three bits to get the number seven
00:15:25 - because zero
00:15:28 - this, hang on. Now now I'm messing with my own head. This is one
00:15:31 - two and four right there so if we add them all up that would
00:15:35 - be zero through seven if you will. Because that's a seven
00:15:39 - of total. So it says well eight is the maximum number
00:15:44 - that you can get with three bits. Sorry, I don't know if that makes sense at all
00:15:48 - but meaning when you have
00:15:51 - three bits the biggest number you can get is eight hosts. Now
00:15:54 - here's the trouble with the hosts. Any time we're figuring out these
00:15:58 - these calculations we always have to subtract two because
00:16:02 - one of those hosts will be the broadcast. One of them will be
00:16:05 - the network. So when we're looking at this it shows zero through seven
00:16:08 - well that gives us eight total values that we can have but if we
00:16:12 - subtract two from eight were left with six. So while we wanted
00:16:16 - seven hosts, that was our requirement give me seven hosts, this
00:16:20 - would only give you six total values once you subtracted the two.
00:16:24 - We've come up one short. So I don't need to get into all the technicalities
00:16:29 - of why this is the way it is. To play it safe this is this is
00:16:33 - the rule always subtract one when finding the number of networks.
00:16:38 - Meaning, if the problem or scenario that you're trying to
00:16:41 - solve says I want you to figure out a 128 networks
00:16:45 - networks well subtract one and figure it out for a hundred twenty
00:16:48 - seven. Or if it's saying you know I want you to figure out thirty
00:16:51 - networks subtract one and figure out for 29. It will
00:16:55 - never hurt you
00:16:57 - to do this on whatever calculation you're trying to figure
00:16:59 - out. It will only help you because it will avoid all of these
00:17:03 - specialized scenarios.
00:17:06 - Chances are very slim that somebody's going to come up and
00:17:08 - say, hey I want you to figure out how, you know, give me a
00:17:11 - network range for 63 hosts per network. I mean, that's pretty
00:17:15 - weird that somebody would be looking for that value. But nonetheless
00:17:18 - we can use this rule always subtract one when finding the number
00:17:22 - of networks and to play it safe with the host always add one
00:17:26 - when finding the number of hosts. So if they ask for 63
00:17:29 - hosts, you know because of this rule you're going to come up one short
00:17:32 - so just add one to the number. Say well, I'll figure it out for
00:17:35 - 64. Someone says I one 20 hosts, add one and figure
00:17:38 - out for 21. It's going to end up with the same answer
00:17:41 - if it's not hitting this rule or if it's this rule it may save
00:17:45 - you a miscalculation. So this is the great exception and I fully
00:17:48 - describe this and document it in that document that I just showed
00:17:51 - you. Probably a little little more concise than the way
00:17:55 - I've explained it because it's hard when you're trying to show
00:17:56 - the binary and how that works but that's the only time the three
00:18:01 - steps will lead you in a little different direction is if one
00:18:05 - of these values are chosen for the networks or one of these
00:18:08 - values are chosen for the hosts. With that I will close the
00:18:13 - door on IP subnetting. That is the subnetting and the skill that you
00:18:17 - will need when you're working with anything in the CISCO world
00:18:21 - because as a CISCO technician you have become the network
00:18:24 - designer. Sure just about every other certification Microsoft
00:18:28 - Novell, they all talk about subnetting but you can kind of
00:18:33 - skate your way through without really knowing what subnetting is
00:18:36 - about because you don't usually deal with it in the Microsoft and Novell
00:18:40 - world. You usually are getting your IP addresses from the CISCO designer
00:18:45 - that figures out all the networks and divides them up on
00:18:48 - a map and so on so it's not as critical to those technologies.
00:18:52 - In CISCO you are the network architect so you have to know
00:18:56 - how to figure out the subnets and reverse engineer them. So to hit the high points,
00:19:01 - three styles: networks, hosts and reverse engineering. I would
00:19:06 - make sure that you are able to figure those out within a decent
00:19:11 - amount of time if you're planning on taking the certification
00:19:13 - exam. And the key to these subnetting systems are practice. I know,
00:19:20 - it's brutal, but that is the only way that this will get
00:19:23 - down in your own mind so if you want you can come up with many
00:19:27 - different scenarios for your own practice or go
00:19:31 - on google and type in subnetting. Be warned though, you will see
00:19:36 - 5,000 different methods of subnetting and once you
00:19:39 - try and start mixing many of those methods into your mind
00:19:42 - it can just get confusing. So just look on on-line for examples
00:19:47 - for some sample questions that you can work through. I hope this
00:19:50 - has been informative and I'd like to thank you for viewing.

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Jeremy Cioara

Jeremy Cioara

CBT Nuggets Trainer

Cisco CCNA, CCDA, CCNA Security, CCNA Voice, CCNP, CCSP, CCVP, CCDP, CCIE R&S; Amazon Web Services CSA; Microsoft MCP, MCSE, Novell CNA, CNE; CompTIA A+, Network+, iNet+

Area Of Expertise:
Cisco network administration and development. Author or coauthor of numerous books, including: CCNA Voice 640-461 Official Cert Guide; CCNA Voice Official Exam Certification Guide (640-460 IIUC); CCENT Exam Prep (Exam 640-822); CCNA Exam Cram (Exam 640-802) 3rd Edition; and CCNA Voice 640-461 Official Cert Guide.

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