Cisco CCNA ICND2 640-816

Subnetting: Understanding VLSM

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

00:00:00 - It's time to move in to everybody's favorite topic and that is
00:00:05 - TCP/IP subnetting.
00:00:07 - When CISCO split the CCNA into two separate certifications,
00:00:11 - the CCENT and the CCNA they also split the subnetting concepts. In the
00:00:17 - CCENT they introduce subnetting and they even got pretty complex with it, but
00:00:22 - they didn't go all the way. Meaning there was one piece of subnetting
00:00:26 - that CISCO left out of the CCENT testing objectives,
00:00:30 - and that is known as variable lengths Subnet masking, or VLSM.
00:00:33 - This is going to be where we talk about VLSM in the
00:00:37 - ICND part two. But I want to make sure that
00:00:42 - is familiar with the ICND part one form of subnetting. It's,
00:00:45 - it's a method of subnetting that i've been using for quite
00:00:48 - some time. It's pretty simple but, I also realize that if you haven't
00:00:52 - gone through ICND part one you're going to be completely
00:00:56 - confused when you see the style of subnetting that I used here. So what
00:01:00 - I what I have right here is my first objective to talk about is to
00:01:03 - remind you all that we have as an appendix to this series all
00:01:08 - the subnetting videos that I created for ICND part one. So if
00:01:13 - you have not seen ICND part one or it's been a while and you
00:01:16 - need to review before we get into the advanced subnetting, feel free
00:01:19 - to look through those appendix videos and check those out. Also
00:01:23 - with those videos is tons of practice for you, so you can
00:01:26 - actually practice through some some questions on your own and then
00:01:29 - review yourself in the answers. Those practice questions and
00:01:33 - answers are going to be available on
00:01:37 - under it'll probably be under the ICND part one's video series
00:01:41 - you can download those, no log in required no cost to you at all
00:01:45 - to work through and practice them so what we're going to do
00:01:48 - here is the second bullet. Working through a VLSM
00:01:52 - scenario. VLSM stands for variable length Subnet mask. It's
00:01:59 - a technical term but really a simple definition. All it means
00:02:03 - is that you can change your subnet mask whenever and wherever you
00:02:07 - want on the network. Now remember what subnetting is. Subnetting
00:02:11 - is taking one network and breaking it into many networks.
00:02:15 - For example, we have
00:02:18 - here, and I could break that into mini networks to address
00:02:22 - this, this network that you see in the diagram. But notice the
00:02:26 - statement, it says subnet to
00:02:31 - address this network, using the most efficient addressing possible. Whenever
00:02:37 - you see most efficient addressing possible that means we're going
00:02:41 - after variable length Subnet masking, which means you're going use
00:02:44 - custom Subnet masks for every segment of the network. Now,
00:02:49 - VLSM in my opinion is no more difficult even though it's the
00:02:53 - most difficult of the CCNA series. I don't really think it's
00:02:56 - that much more difficult than any other subnetting problem
00:02:59 - that we've seen thus far. If you look through the appendix videos but
00:03:03 - what I will say is it will take longer because it's multiple
00:03:06 - Subnet problems in one. So let's work through this scenario
00:03:10 - one network at a time.
00:03:12 - So, what I'm going to do is squish that network diagram up in the corner so we have some
00:03:16 - working space here, and look at the scenario one more time.
00:03:19 - We need to Subnet to address the
00:03:22 - network using the most efficient addressing possible. Now whenever
00:03:26 - you see that think VLSM and then think the whole key
00:03:30 - behind VLSM is to start
00:03:35 - with the largest Subnet. Now looking at that diagram what
00:03:43 - is the largest subnet?
00:03:45 - That one, 60 users right there. So we need to start with that one,
00:03:49 - now this is using the methods that I was talking about in
00:03:52 - the videos and in the appendices, so again if you haven't seen those
00:03:54 - videos now's the time to check them out. So we've got 60
00:03:58 - users. I'm going to take my 60 users and convert that to
00:04:02 - binary. Put up our binary chart, 128 64
00:04:05 - 32 16 8 4 2 1. 60 is a one in the 32
00:04:13 - one in 16 that's 48 add another eight that
00:04:18 - would be
00:04:19 - 50. I shouldn't have done this in my head. 48 plus eight
00:04:23 - a is 56 so add a four and we've got our 60. So there's
00:04:28 - 60 in binary. Zero zero one one one one zero zero. Now you
00:04:34 - might remember that we're not... After the exact binary number
00:04:38 - we're mainly concerned with how many bits it took to get the
00:04:41 - number 60. The number of bits is six, so we can't get the number
00:04:45 - 60 with any less than six bits. So, move on to step two.
00:04:50 - Step two is to write the original Subnet mask in all binary. So a
00:04:54 - /24 is 24 ones. So a whole bunch of ones
00:04:57 - that's, imagine that being 24 of them. And then eight zeros.
00:05:01 - One two three four five six seven eight. Now we've got
00:05:05 - 60 that we converted to binary are we after creating more networks
00:05:09 - here or saving the hosts. We're looking at 60 after the
00:05:14 - users I need to save some hosts. So I take right to the left
00:05:18 - one too three four five six the other two can become ones.
00:05:27 - I've saved my six zeros. So with that in place my new Subnet mask
00:05:31 - for the 60 user network will be slash 26 or two
00:05:37 - 255.255.255 and this in
00:05:42 - decimal over here is dot 192. The lowest network
00:05:46 - bid is my increment. That's that guy it is a 64.
00:05:51 - So, I come down to my third step and I can find my network ranges. So
00:05:56 - I go that's where
00:06:00 - I began and then I'll just go to 1.64 and actually stop
00:06:06 - right there. How many networks of 60 users do I need?
00:06:10 - Looking at this diagram there's just one. So I can fill in
00:06:13 - the end range that would be
00:06:17 - through 63 and there is my network range. What I'll do is
00:06:21 - on my network diagram I'll notate as matter of fact, let me type it
00:06:25 - so it's a little neater. Right here will be
00:06:30 - 192.168, change that font just a moment.
00:06:35 - .1.0 through 63 slash, and I'll put my Subnet mask,
00:06:41 - 26.
00:06:43 - Let me increase that font size a little bit. There we go. I'll drag
00:06:48 - that right here. Good so we've got that which is now my first
00:06:53 - VLSM subnet.
00:06:56 - With that in place the whole key to VLSM is now do it
00:07:00 - all over again. So I'm going to take what I've done
00:07:05 - oops, the one Subnet I have there. Let me select the right field there.
00:07:11 - There we go. Erase it and start all over now you can see I've got one subnet
00:07:15 - solved. Now let's move to the next biggest, you see VLSM you start with
00:07:19 - the biggest Subnet and you work your way down. So I'm moving on
00:07:22 - to the next biggest which is 20 users. So we'll take twenty, convert
00:07:26 - that to binary. Oh, should've saved the binary chart. Save the chart. So
00:07:31 - 32, 16, 8, 4, 2, 1. And I'm going to say 20 in binary
00:07:36 - is zero. Oh, here is my first one right here. One good grief.
00:07:42 - One and one the rest of these are zeros so 20 in binary is zero zero zero
00:07:48 - one zero one zero zero. It takes five bits to get the number 20.
00:07:56 - So step two is now to take my original Subnet mask /24
00:08:00 - write that in binary, 24 ones and we're writing this
00:08:04 - as if I had never done any subnetting in the first place.
00:08:07 - There's my eight zeros. Now I'm, again after saving my host because I am
00:08:12 - after 20 users, five host bits so right to left one two
00:08:17 - three four five remain zeros, the rest can become one my new
00:08:21 - Subnet mask for that portion of my network is /26
00:08:26 - or wate, no, /27 because there's 27 ones
00:08:29 - or in decimal it would be two five five dot two five five dot two five five
00:08:34 - and that would be.224
00:08:37 - Lowest network bid is my increment. Convert that back to decimal.
00:08:40 - That would be a 32. So step three,
00:08:46 - is to start off with what I was given 192 168
00:08:50 - Just as if I had done nothing and start adding 32
00:08:54 - 1.32, 1.64. Now wait
00:08:59 - just a second. Let's fill in some end ranges here. I'll go through
00:09:03 - 31 through 63. Now can I use any of these?
00:09:10 - No. Cause they're already used right here. You see that one is zero to 63
00:09:14 - three so that is unusable. That is the unusable. I need to
00:09:18 - start off with where I left off which is 64 and keep
00:09:21 - adding 32. So 96
00:09:25 - .128. Now I only need two networks of 20 users each
00:09:30 - so I can go in there and just stop after these two ranges.
00:09:33 - This will be through 95 and this will be through
00:09:36 - 127. You're seeing kind of how this works? So I picked up
00:09:41 - where I left off. Zero through to 63 is used up right here. I
00:09:45 - have, let me grab my
00:09:49 - text tool. Going to copy this guy up here so we'll notate.
00:09:56 - or this will be let's change
00:10:00 - that, 64 through 95/27. Good.
00:10:08 - And then I'll assign that second range over here to these 20
00:10:11 - users. This will be 192.168.96
00:10:16 - through 127/27. Good.
00:10:21 - So now we've got both of those subnets completed out
00:10:24 - there the 20, the two subnets of 20 users and if you look,
00:10:29 - well there goes all our work again. It's gone because we've now
00:10:33 - addressed the 60 users and the two networks of 20 users
00:10:36 - but there's still one more that's very easy to forget. You see
00:10:40 - the WAN links. How many users are on a point to point WAN link?
00:10:44 - Two. There'll never be any more than two users on a point to point
00:10:48 - WAN link. That's the definition of point to point. So I'll start over again
00:10:52 - for my last network range and that is two users. So, two users. Let me put my
00:11:00 - binary chart back up there.
00:11:02 - One day I'll learn not to wipe it out. 8 4 2 1. Two users
00:11:06 - that would be one zero so it takes two bits or zero zero zero zero
00:11:11 - zero zero one zero. Two bits to get the number two.
00:11:16 - So I write my original Subnet mask in all binary. /24
00:11:21 - whole bunch of ones, 24 of them, dot one two three
00:11:24 - four five six seven eight zeros. Now I'm saving the host because
00:11:29 - I have two users so those two remain zeros the rest of them
00:11:33 - become ones so I have six ones and two zeros leftover. My subnet
00:11:38 - mask moves to a slash 30 or if I were writing in decimal, it would be two five
00:11:44 -
00:11:49 - two fifty two. My increment, lowest network bit converted back
00:11:53 - to decimal is four. So now I go to my step three which is finding
00:11:58 - my network ranges. I can go
00:12:02 - zero dot one dot four dot one dot eight. Now,
00:12:06 - hang on just a moment. Can I use any of these zero four, zero through three, four
00:12:11 - through seven? No. Because they're already used up right here. As a matter
00:12:15 - of fact I could keep counting back 4 12 16 20
00:12:18 - twenty and so on and I'd go through all these ranges and all of these
00:12:21 - and they're all used up. Now if you if you don't feel comfortable
00:12:25 - doing this, don't but what I would say is it's a lot easier to
00:12:28 - just go dot dot dot and pick up where you leaved off. You notice a right here
00:12:33 - we ended at 127 so the next one would be dot
00:12:37 - 128.
00:12:42 - 132 136 140. If you keep counting by
00:12:49 - a four you'll eventually get to 128. And you don't want to do that
00:12:52 - as efficiently as possible without filling the page with
00:12:55 - numbers. So I can now fill in my end range, that would be 131,
00:12:58 - one, one thirty five, one thirty nine. So, let me
00:13:06 - grab my subnets I have been using right here. Copy and paste them I will
00:13:10 - assign this WAN link 192.168.120 twenty
00:13:15 - through 131/30.
00:13:21 - There we go. We'll use over here one thirty two through one thirty
00:13:32 - 135/30. And then finally up here
00:13:40 - one thirty six through one thirty nine slash thirty. And that
00:13:47 - is addressing a complete network using VLSM.
00:13:51 - Now, if you look at this it's pretty amazing because we've addressed
00:13:54 - this whole network and barely used over half our network range. If
00:13:58 - we didn't have VLSM
00:14:02 - we, we couldn't do this. I was just thinking of that. We, this would
00:14:04 - be impossible because we'd have to -- if we had -- could only use
00:14:07 - one Subnet mask which is what VLSM is trying to prevent
00:14:12 - If we could only use one Subnet mass we have to figure it for the biggest one.
00:14:16 - And if you go in increments of 64 you'd only get four networks
00:14:20 - and you can see we would need more than four networks to complete
00:14:22 - that scenario. So VLSM is the only way that we could effectively
00:14:26 - address that network and it is very efficient. Now, there's a couple
00:14:29 - notes want to add onto this before, before we wrap things
00:14:33 - up. The first thing I'll mention is why we started with
00:14:37 - the largest Subnet first. If you think about it if we would have
00:14:41 - started with the smallest or, or some other Subnet first
00:14:44 - there would be waste. Notice it says use the most efficient addressing
00:14:48 - possible. Well let's say we started with this WAN link Subnet
00:14:51 - right here. Zero through three four through seven eight through
00:14:54 - that would be through eleven if we were to continue that for
00:14:57 - WAN links. So that would be if we started with the smallest and that
00:15:00 - would kill this this first group up to the IP address one
00:15:04 - Now, let's say we move to this 60
00:15:08 - user Subnet after that. Well our first range would be zero
00:15:11 - through 63 but we couldn't use that because these were
00:15:15 - already used up. The WAN links already killed zero through 11
00:15:20 - which would overlap so we would have to go with 64
00:15:23 - through, what, that would be 127 for this range right
00:15:26 - here which you would have a waste. You would waste from 12 through
00:15:30 - 64 because you started with the smaller Subnet rather
00:15:34 - than starting with the largest Subnet. So that is why we start
00:15:37 - with the smallest one first. The second thing I want to mention
00:15:42 - while I've got the diagram up is VLSM like this looks
00:15:47 - beautiful on paper.
00:15:50 - But think of real world with me here. If this Subnet
00:15:55 - up here our 20 user Subnet had a sudden hiring spree and
00:15:59 - brought on 50 new employees well that would blow up your
00:16:02 - whole IP addressing scheme because you would exceed your
00:16:05 - 20 users Subnet, which only handles up to about 30 users
00:16:09 - and you have to re- address the whole network so when you're
00:16:12 - doing this in the real world always use room for growth.
00:16:16 - So, so don't Subnet it so tight to where it looks perfect on paper
00:16:20 - but only lasts a good year before you have to re-address the whole
00:16:23 - network anyway.
00:16:24 - And, let's see what else can I mention about this? VLSM
00:16:29 - is very commonly used right here on the WAN links, most
00:16:33 - of the organizations that I've seen on all the point to point
00:16:36 - WAN links will use the /30 and on the LAN they'll use something
00:16:40 - easy like a /24. Even doing something that simple
00:16:44 - is using VLSM
00:16:46 - that leads to the big point which means if you use
00:16:50 - VLSM you must have a class less routing protocol and
00:16:57 - those include RIP version two OSPF, EIGRP ISIS and
00:17:04 - that's it that I can remember off the top of my head.
00:17:08 - The ones it does not include are the two class full protocols let's
00:17:11 - go that way. Class full is RIP version one and the old IGRP
00:17:16 - which CISCO no longer manufactures. So classless routing
00:17:20 - protocols only will support that kind of environment.
00:17:24 - So, that is variable length Subnet masking. Not to bad, right?
00:17:27 - It's just multiple Subnetting problems in one. Yes it does take a
00:17:31 - little bit more time to do but it's really, once you get
00:17:36 - past the idea of doing more than one Subnetting problem it's not too bad.
00:17:38 - Now, if you're preparing for the exam you might be wondering
00:17:42 - how would they ask a question like that on the test? Well what I
00:17:46 - would say is think drag and drop. Think of having a picture of a
00:17:51 - network diagram and all these blanks where you can put Subnets,
00:17:54 - and then they'll say Subnet this for the best possible or
00:17:58 - most efficient way and have all these options of Subnets
00:18:01 - over on the left inside that you can drag and drop to complete
00:18:04 - the network diagram. So when you're doing it that way sometimes
00:18:07 - you may find it more efficient depending on your skills to
00:18:10 - reverse engineer all the Subnets on the left hand side
00:18:13 - to find what would be the what would you know satisfy the requirements
00:18:16 - or you may find a more efficient to do what I just did and
00:18:19 - do a complete DLSM prompt. It's your choice. So we saw first
00:18:23 - off that there are more Subnetting videos into the apendeces since
00:18:26 - my recap here. If you do need more Subnetting practice and
00:18:31 - then we worked through a complete VLSM scenario using one
00:18:34 - class C Subnet with multiple Subnet masks to address the
00:18:37 - whole network. I hope this has been informative for you and I would like
00:18:40 - to thank you for viewing.

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WAN Connections: Implementing PPP Authentication

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WAN Connections: Configuring Frame Relay

IPv6: Understanding Basic Concepts and Addressing

IPv6: Configuring, Routing, and Interoperating

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Advanced TCP/IP: IP Subnetting, Part 1

Advanced TCP/IP: IP Subnetting, Part 2

Advanced TCP/IP: IP Subnetting, Part 3

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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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