Original subtitles

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welcome back, my name is David Bombal CCIE 1123

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and in this section, we're going to look at Variable Length Subnet Mask or VLSM

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and route summarization.

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VLSM allows us to better use IP addresses

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by allowing for Variable Length Subnet Mask on a single network.

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so what we're going to cover in this section is firstly VLSM

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once again Variable Length Subnet Mask, we're going to look at CIDR

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or classless Inter-Domain Routing

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I want to explain summarization and show you

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how you can summarize multiple routes into fewer or single a route.

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we're going to look at routing choices

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and how routers choose one route over another

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not just based on administrative distance

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but also based on the legnth of the prefix match

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and then lastly we're going to look at issues regarding discontiguous networks.

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so Variable Length Subnet Mask

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allow us to have varying or variable mask throughout our network

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so for example, a classful network of 10

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will have a mask of /8 with the first octet

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10 is the network portion of the address

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and the last 3 octets, in other words, the last 24 bits

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is the host portion of the address.

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that classful network was subnetted

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we could, for instance, have a subnet of 10.1.1.0/24

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where 10.1.1 is the network portion of the address

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in other words, the most siginificant 24 bits

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or most significant 3 octets is the netwok portion and the last octet

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or least significant portion of the address is the host portion.

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we could also subnet a 10 network down to 10.1.0.0/16

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where half of the address is the network portion

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and half of the address is the host portion.

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please refer to the ICND 1 part of this course

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or my subnetting explained e-book, for lots of detail on subnetting.

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CIDR or Classless Inter-Domain Routing was introduced in 1993

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to replace the prior addressing architecture of classful networks.

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classful networks were not scalable

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classful networking introduce 3 classes of addresses

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for allocation of IP addresses to hosts and networking devices

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we had A B and C networks.

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so for instance 10.0.0.0/8 is an example of a classful A network.

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172.16.0.0/16 is an example of a class B network

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and 192.168.1.0/24 is an example of a class C network.

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the problem with this method is that you were forced to use

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for instances a /16 mask which gave you approximately 65000 host addresses

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or you could use a class C address like this

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which only gave you 254 host addresses.

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this scheme would have resulted

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in the quick extortion of IP addresses and inflexibility.

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so CIDR replaced this which is based on Variable Length Subnet Mask

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were masks can vary on arbitrary lengths

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so they allow for different size subnets

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so rather than being forced to use for instance 10.0.0.0/8

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you can also use 10.0.0.0/16 or 24 or 30 depending on the requirements

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for the number of subnets or hosts in your network

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or you can use for instance 10.1.1.0/27 or 10.1.2.0/26 and so forth and so on.

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so you could take a single class A address

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and subdivide it into multiple subnets based on your requirements.

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it's also important to note that in CIDR

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we don't just advertise a network, we advertise a routing prefix

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in other words, we would advertise 192.168.1.0/24 not just 192.168.1.0

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CIDR also allows for the summarization of addresses

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which I'm going to explain in more detail in a moment.

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but this is were we take multiple subnets

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and put them into a super net or summary network

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which will reduce routing table sizes.

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Variable Length Subnet Mask have many advantages

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including the better utilization of IP addresses and subnets.

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on a WAN link as an example only 2 IP addresses are required.

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1 IP address for 1 end of the link

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and another IP address for the other end of the link

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thus a /30 mask would be better in say a /16 or /24 mask.

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a /30 mask only support 2 hosts on that subnet

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and thats all that's required in the point-to-point scenario like this.

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a /24 mask for instance would support 254 hosts

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and that would be over kill on this specific link.

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so VLSM gives assist the administrator

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the ability to better utilize IP addresses and subnets.

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where we subnet down subnets

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to support the required number of IP addresses on that specific segment.

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so as an example, here we have subnets

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that are using a /27 mask, on the WAN links we are using /30 mask

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and for the instance at the headquarters we are using a /16 mask.

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it also allows us to implement better summarization

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and scalability of networks as rather than the entire world

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being forced to use a specific subnet mask

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we are able to use varying mask in our own networks

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and advertise summaries to other companies or to the Internet.

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so heres a simple network to explain the problem

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when summarization is not used.

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on the left hand side we have networks 10.1.1.0/24

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up to 10.1.2.200.0/24

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on the right hand side we have networks 10.2.1.0/24

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up to 10.2.200.0/24 now summarization is now implemented

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and let's say you're running a routeing protocol like RIP

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which advertises the entire routing table every 30 seconds

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you're going to have scenario like this

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with the router on the left-hand side

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advertises 200 routes every 30 seconds and the router on the router side

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also advertises 200 routes every 30 seconds.

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RIP will continually sends out its entire routing table

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every 30 seconds even though there are no changes.

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so 400 routes are advertised across this WAN link

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which is very inefficient, those routing updates

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consume a lot of bandwidth and don’t accomplish much

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because they will continuously be advertise

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even though there are no changes

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so it makes more sense to summarize the routes

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so on the left-hand side, we have 10.1.1.0

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all the way up to 10.1.200.0

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So these are subnets of the 10.1.0.0 network

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so we could summarize those 200 routes into a single route

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and on the left hand side 10.2.1.0 up to 10.2.200.0

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could be summarize as 10.2.0.0/16

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so rather than advertising 400 routes a single route from the left-hand side

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is advertised to the left-hand side.

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and a single route from right-hand side is advertised to the left-hand side

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With summarization what we are have looking for

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is most significant bits that are equal, in other words

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starting from the left-and side and working towards the right-hand side

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we look for bits that are the same

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so in the first octet all the subnets contain 10

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so that’s common, in the second octet all the subnets contain 1

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so that's common but in the 3rd octet the values vary from 1 - 200

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so it’s not as easy to see what’s in common

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so we could just summarize it as 10.1.0.00/16

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knowing that 10.1 is common throughout

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and the router over here will summarize all of those routes

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and only send out 1 advertisement

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the router on the left-hand side can still get to all of this subnets

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because they are subnets of this network

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so even though we've summarized routes

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we still have full connectivity throughout the network

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here’s some more complicated example

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assume that we have networks 172.16.32.0/24

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all the way up to 172.16.63.0/24

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now rather than this router advertising all of those networks

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can we summarize this network into 1 or at least a few summaries

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now to work this out you start from the most significant bits

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in other words, you start from the left-hand side

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and you look for what is common throughout all of this networks

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so in the first octet 172 is common, so we know that’s common

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16 is also common, so we know does far that the 1st 2 octets are common

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in the third octet, however, the numbers are changing

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so we have 32, 33, 34, 35, all the way up to 63

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so it's not as easy to visualize or see what’s common here

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so what we're going to do is we can convert the third octet

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into binary to look for common bits, so for example 32 = 0010 0000

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now obviously there’s no gap halfway through an octet

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I’ve just put the space here to try and make it easier to read

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so an octet consists of 8 binary values, 32 would look at follows

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now as soon as you convert into binary

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you should convert the remaining bits into binary as well

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now in this example, you don’t need to do that

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because you can see that the last octet contains just a 0

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but I’ve done it here for completeness, the second network is 172.16.33.0

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and 33 in binary looks as follows, 34 looks as follows, 35 looks as follows

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and I wouldn’t recommend that you convert all the addresses into binary

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I would just do say the first 3 or 4

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and then the last one to see what’s in common.

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in the real world, however, you may come across far more complicated examples

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and you may need to convert all the addresses into binary

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but at the CCNA level that may not be necessary.

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So once we’ve done that we can look for bits that are in common

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so the first binary 0 in the 3rd octet is common throughout

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the second binary 0 is also common throughout, the 3rd binary bit is a 1

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and that is common throughout, however, in the 4th binary bit position

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the values change as you can see here the first few networks have 0's

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but the last network has a 1 in the fourth binary bit position

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so we can draw a line to the right of all the bits that are common

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so as you can see here to the left of the line in the 3rd octet

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we have 001 in binary

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the remaining bits in binary are not common in other words bits vary.

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Now Cisco does not support discontiguous subnet masks

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so you can see these bits are not in common and these this bits are in common

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there has to be a contiguous grouping of common bits

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starting from the left-hand side moving from the right-hand side

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and as soon as there are bits that are not common

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you have to draw a line to make a differentiation between common bits

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and bits that are not common

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so we know now that the first octet is common, the second octet is common

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so 172.16 in the third octet, the first 3 binary bits are common

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so 001 now any bits that are not in common are just set to 0

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so notice, we’ve populate the remaining portion of the address with binary 0's

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and then we convert the binary back to decimal

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so hopefully remember this from the ICND 1 course.

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So the first octet is 172, the second octet is 16

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the third octet if you convert this to decimal is equal to 32

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and the last octet converted to decimal is 0.

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So the summary address is 172.16.32.0

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The last step is to work out the bits that are in common

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now the first octet is in common and an octet is 8 bits

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the second octet is in common and that’s an additional 8 bits

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so thus far we have 8 bits + 8 bits in other words, 16 bits that are in common

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3 bits in the third octet are in common, so 8+8+3 will give you 19 bits in common

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thus the summary address for this subnets will be 172.16.32.0/19

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it’s a simple as that to work out summarization

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and I’m going to show you trick in a moment

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that allows you to work this out in the few seconds

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here’s another example, we have subnets 172.16.64.0/24 up to 172.16.127.0/24

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can these subnets be summarized into a single subnet or fewer subnets?

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so using the same process we start from the left-hand side

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and we look for bits that are in common

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in the first octet, we have 172 throughout all of those subnets

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so that’s common, in the second octet we have 16

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so that’s common throughout

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So we know that the first 2 octets 172.16 are common throughout

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but in the third octet the values are changing

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we’ve got 64, 65, 66, 67 all the way up to 127

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so that third octet we're going to convert to binary

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to be able to better see what’s in common

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so converting 64 into binary will give you 00100 0000

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65 will look as follows, 66 as this, 67 as that

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and you could convert a third octet of all of the subnets until you get a 127

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which looks as follows

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the last octet is a 0 in decimal which looks as follows in binary

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so now what we need to do is we need to look for the common bits

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so the common bits are once again 172.16

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and then the first binary 0 is in common throughout the subnets

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the second binary bit which is set to 1 is common throughout

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but the third binary bit is not common throughout notice there’s 0

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and then here there’s a binary 1

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so we can draw a line after the second binary bit

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to denote that everything to the left of the line is in common

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and everything to the right of the line is not in common

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so all of this is in common and all of this is not in common.

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therefore 172.16 in decimal, the first 2 octets are in common

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and the first 2 binary bits of the third octet are in common

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the remaining binary bits need to be set to 0's

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so filling the remaining binary bits is to 0 will look as follows

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and then converting the binary back to decimal

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will get 172 in the first octet, 16 in the second octet

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this is 64 in decimal, so the third octet is 64

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and the last octet binary 0's is equal to 0 in decimal

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so the address is 172.16.64.0 that is the summary network for all of these subnets.

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the last step is to count the number of bits in common

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the first octet is 8 bits, the second octet is 8 bits

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so that’s 16 bits in total followed by another 2 binary bits

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which gives you 18 bits in common

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so the first 18 bits are in common throughout all of this subnets

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so our final answer will be 172.16.64.0/18

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that is the summary network for all of the listed subnets.