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CCNA v3.0 RetoolingWhat is VLSM?
A Variable Length Subnet Mask (VLSM) is a means of allocating IP
addressing resources to subnets according to their individual need
rather than some general network-wide rule.
VLSM allows an organization to use more than one subnet mask within
the same network address space. It is often referred to as
‘subnetting a subnet’, and can be used to maximize addressing
efficiency.
Large subnets are created for addressing LANs and small subnets are
created for WAN links (a 30 bit mask is used to create subnets with
only two host).
CCNA v3.0 Semester 3
Subnetting vs. VLSM
Subnetting allows you to divide big networks into smaller,
equal-sized slices.
VLSM allows you to divide big networks into smaller,
different-sized slices. This enables you to make maximum use of
your valuable IP address space.
So basically, you are now utilizing subnet masks in the same IP
address space.
CCNA v3.0 Semester 3
Routing Protocols Supporting VLSM
207.21.24.0 /24
How many hosts are needed for the largest LAN?
How many bits need to be borrowed to address this network?
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Site A has two Ethernet networks
Site B had one Ethernet network
Site C had one Ethernet network
If we borrow 3 bits from a class C address, that will give us eight
networks, but we can only use six of them. Each network will have
30 usable addresses.
It will take four network addresses to accommodate the Ethernet
networks at each site. That leaves us with two extra
networks.
There is also a point-to-point WAN connection between each site.
These two connections will take up the remaining two
networks.
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Addressing a Network with Standard Subnetting
Borrowing 3 bits will meet the current needs of the company, but it
leaves little room for growth.
Each network will have 30 usable addresses, including the
point-to-point WAN links (which only require two addresses).
207.21.24.0
We can use subnet 0
To enable subnet 0 on a Cisco router (if not already enabled), it
is necessary to use the global configuration command ip
subnet-zero.
Router# configure terminal (config t)
Router(config)# ip subnet-zero
To disable subnet 0, use the no form of this command.
Router(config)# no ip subnet-zero
CCNA v3.0 Semester 3
Subnetting in a Box
Provides 1 network with 256 addresses.
When we subnet the address, we break it down in to smaller units or
subnets.
Subnet mask: 255.255.255.0
0
255
Borrowing 1 bit would break the 256 addresses in to two parts
(networks)
Providing 2 networks each with 128 addresses.
Subnet mask: 255.255.255.128.
0
255
Borrowing 2 bits would break each of the 2 networks in half
again.
Providing 4 networks, each with 64 addresses.
Subnet mask: 255.255.255.192.
0
255
Borrowing 3 bits would break each of these 4 networks in half
again.
Providing 8 networks, each with 32 addresses.
Subnet mask: 255.255.255.224.
0
255
Borrowing 4 bits would break each of these 8 networks in half
again.
Providing 16 networks, each with 16 addresses.
Subnet mask: 255.255.255.240.
Addressing a Network Using VLSM
When using VLSM to subnet an address, not all of the subnets have
to be the same size.
A different subnet mask may be applied to some of the subnets to
further subnet the address.
In order to take advantage of VLSM, the proper routing protocol
must be selected.
Not all routing protocols share subnetting information in their
routing table updates.
Sheet1
Classless Routing Protocols (do share subnet info)
RIP v1
RIP v2
Addressing a Network Using VLSM
To subnet using VLSM, identify the LAN with the largest number of
hosts. Subnet the address 207.21.24.0 /24 based on this
information.
Site A has two Ethernet networks (25 hosts each)
Site B had one Ethernet network (10 hosts)
Site C had one Ethernet network (8 hosts)
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Subnet 1 & 2 to address Site A Ethernet networks.
Subnet 5 to accommodate Site B & C Ethernet networks.
Subnet 6 can be subnetted to accommodate the WAN links.
Free Addresses
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Site A
Addressing a Network Using VLSM
Through applying VLSM, the topology was able to be addressed and
still have two complete subnets available for future growth.
207.21.24.32 /27
207.21.24.64 /27
207.21.24.160 /28
207.21.24.176 /28
207.21.24.192 /30
207.21.24.196 /30
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Exercise 1
Your company IP network is 195.39.71.0 /24.
Headquarters is connected to five branch offices by a WAN link, and
to an ISP.
Determine an appropriate IP addressing scheme.
(the ISP owns the addresses on its link)
Headquarters
Subnet according to the largest subnet needed. (Headquarters 60
hosts)
0
255
Borrow 2 bits or /26. This would give you 4 networks with 64 host
addresses on each subnet.
128
127
64
192
63
191
CCNA v3.0 Semester 3
Playing it safe, we will not use the first subnet (subnet 0).
We will start addressing with 195.39.71.64 /26.
Headquarters needs 60 hosts, so we will assign them .64 -
.127.
Headquarters
(255.255.255.192)
0
64
128
192
The 5 Branch offices need 12 hosts each.
The next address block available is the .128 - .191 block. Use
VLSM.
Headquarters
(255.255.255.192)
Using a /28 mask will give us 16 hosts at each location. This will
take care of 4 of the Branch offices.
0
64
128
192
160
144
176
CCNA v3.0 Semester 3
To obtain a block for Branch 5, we will need to subnet the .192 -
.255 block using a /28 mask.
Headquarters
(255.255.255.192)
Now connect the 5 WAN links to the Branch offices.
These are point-to-point connections and only require 2
addresses.
0
64
128
192
Here we will use a /30 mask to further subnet the subnets.
Headquarters
(255.255.255.192)
CCNA v3.0 Semester 3
Any remaining networks could be used for future growth of either
LANs or WANs.
Subnet 0 could also be further subnetted according to the needs of
the network.
0
64
128
192
Headquarters
(255.255.255.192)
Address provided by ISP
8 bit network number for Class A addresses
16 bit network number for Class B addresses
24 bit network number for Class C addresses
1 - 126
128 - 191
192 - 223
Class
Number of Hosts
32
/26
64
/25
128
/24
65,536
/15
131,072
/14
262,144
/13
524,288
/12
1,048,576
/11
2,097,152
/10
4,194,304
/9
8,388,608
/8
65,536 Class C or 256 Class B or 1 Class A
16,777,216
Sheet3
CIDR (pronounced “cider”) ignores class.
Using CIDR, a router views a bit mask to determine the network and
host portions of an address.
This allows CIDR to craft network address spaces according to the
size of a network instead of force-fitting networks into pre-sized
network address spaces.
CCNA v3.0 Semester 3
CIDR is usually discussed in general Internet context (ISPs)
Uses custom length prefixes to reduce workload in key Internet
routers
VLSM is usually discussed in enterprise context
Uses custom length prefixes to have better usage of enterprise
address space
CCNA v3.0 Semester 3
Classless Interdomain Routing
Routers use the network-prefix, rather than the first 3 bits of the
IP address, to determine the dividing point between the network
number and the host number.
In the CIDR model, each piece of routing information is advertised
with a bit mask or prefix-length ( /x ). The prefix-length is a way
of specifying the number bits in the network-portion of each
routing table entry.
CCNA v3.0 Semester 3
Classless Interdomain Routing
For example, a network with 20 bits of network-number and 12 bits
of host-number would be advertised with a 20 bit prefix
(/20).
The clever thing is that the IP address advertised with the /20
prefix could be a former Class A, Class B, or Class C.
All addresses with a /20 prefix represent the same amount of
address space (212 or 4,096 host addresses).
20 bits network + 12 bits host
CCNA v3.0 Semester 3
Classless Interdomain Routing
Address space can now be assigned in “chunks” that fit the
need.
If an organization needs 254 host addresses, what difference does
it make whether they are given:
a Class C (200.23.76.0 /24)
1/256th of a Class B (145.38.20.0 /24)
1/65,536th of a Class A (91.187.7.0 /24)
Using a /24 prefix, each of these specifies eight host bits which
will support 254 hosts.
CCNA v3.0 Semester 3
Number of Hosts
32
/26
64
/25
128
/24
256
/23
512
/22
1,024
/21
2,048
/20
4,096
/19
8,192
/18
16,384
/17
32,768
/16
65,536
/15
131,072
/14
262,144
/13
524,288
/12
1,048,576
/11
2,097,152
/10
4,194,304
/9
8,388,608
/8
65,536 Class C or 256 Class B or 1 Class A
16,777,216
Sheet3
You need 500 addresses.
Given two consecutive /24 addresses:
(200.201.202.0 /24 and 200.201.203.0 /24)
This address space could be advertised to the rest of the Internet
as 200.201.202.0 /23.
Why? (the two /24s have the first 23 bits in common).
11001000.11001001.11001010.00000000
11001000.11001001.11001011.00000000
CIDR Scenario continued
If the ISP owns all of the 200.201.0.0 networks (256 /24s), why
should it advertise all of them separately?
Instead, it could simply advertise 200.201.0.0 /16 (which would be
200.201.0.0 /24 through 200.201.255.0 /24).
This would reduce the size of the routing tables on the router to
which the routes are advertised.
11001000.11001001.00000000.00000000
11001000.11001001.11111111.00000000
CIDR Scenario continued
The summary of route 200.201.202.0 /23 is called a “CIDR block” or
a supernet.
Because we are dealing with binary, the block size is always a
power of two (2, 4, 8, 16, 32, etc.). The starting point of the
block must be a multiple of the power of two that is being used (21
… 2, 4, 6, 8, etc.).
200.201.202.0
200.201.204.0
200.201.206.0
200.201.208.0
200.201.210.0
Hand out pieces of classful networks (to avoid wasting
addresses)
Identify the network portion of an address with a network prefix (
/x)
Advertise blocks of networks (to reduce the size of routing
tables).
CCNA v3.0 Semester 3
Scenario
You are assigned the CIDR address 200.32.108.0 /22 and you must
support the network shown in the diagram. Create an addressing
scheme that will meet the diagram requirements.
300 users
100 users
100 users
100 users
How many /24 networks do we have?
How many host addresses do we have?
What is the largest LAN requirement?
300 users
100 users
100 users
100 users
Host required - 300, 100, 100, 100, and 3 WAN links
200.32.108.0
Host required - 300, 100, 100, 100, and 3 WAN links
0
0
0
0
255
255
255
255
Host required - 300, 100, 100, 100, and 3 WAN links
0
0
0
0
255
255
255
255
Host required - 300, 100, 100, 100, and 3 WAN links
0
0
0
0
255
255
255
255
Host required - 300, 100, 100, 100, and 3 WAN links
0
0
0
0
255
255
255
255
Two /24s
300 users
100 users
100 users
100 users
200.32.108.0 /23
200.32.110.0 /25
200.32.110.128 /25
200.32.111.0 /25
200.32.111.240 /30
200.32.111.248 /30
200.32.111.244 /30
Classless Interdomain Routing
For the router to operate in a classless manner and match
destination IP addresses to a CIDR network address,
The global command: ip classless must be configured.
Router(config)# ip classless
(RIP)
RIP is a relatively old, but still commonly used interior gateway
protocol (IGP).
It was created for use in small homogeneous networks.
It is a distance-vector protocol that is used with classful IP
addressing only.
RIP v1 sends routing update messages at regular intervals (30
seconds) and when the network topology changes.
RIP uses hop count as its only metric and maintains only the best
route to a destination.
CCNA v3.0 Semester 3
Known as RIP V2
In RIP v2 all of the operation procedures, timers, and stability
functions of RIP v1 remain the same in version 2, with the
exception of the broadcast updates.
RIP v2 has become the standard version of RIP used in networks
today.
CCNA v3.0 Semester 3
Authentication of routing updates
External route tags
Multicast route updates
RIP v2
The most important of these extensions is the addition of a Subnet
Mask field
This enables the use of variable-length subnet masks (VLSMs) and
qualifies RIP v2 as a classless routing protocol.
RIP v2 Packet Format
RIP v1 Packet Format
CCNA v3.0 Semester 3
RIP v2
RIP v2 allocated a 4-octet field to associate a subnet mask to a
destination IP address.
When used in tandem, the IP address and its subnet mask enable RIP
v2 to specifically identify the type of destination that the route
leads to.
This allows RIP v2 to route specific subnets, regardless of whether
the subnet mask is fixed or of variable length.
RIP v2 Packet Format
CCNA v3.0 Semester 3
RIP v2
RIP v2 differs from RIP v1 in the way update are
sent out.
RIP v1 sends updates as a broadcast (all stations receive the
broadcast message)
RIP v1 does not send subnet mask information in its updates.
RIP v2 sends updates as a multi-cast. Multi-casting is a technique
for simultaneously advertising routing information to multiple RIP
devices via the class D address 224.0.0.9
CCNA v3.0 Semester 3
Both use hop count as a metric
Both have the same metric value for infinite distance (16)
Both use split horizon to prevent routing loops.
RIP v1 broadcasts routing table updates, while RIP v2 multicasts
its updates
CCNA v3.0 Semester 3
Configuring RIP v1
To configure RIP v1 on a router, enter the following
commands:
Router# config t
Router(config)# router rip
Router(config-router)# network 192.168.12.0
NOTE - If no version is specified in the configuration, version 1
will be used. The router will listen for version 1 and 2 updates
but send only version 1.
CCNA v3.0 Semester 3
Configuring RIP v2
To take advantage of version 2s features, it is necessary to turn
off version 1 support and enable version 2 updates with the
following commands:
Router(config)# router rip
Router(config-router)# version 2
Router(config-router)# network 192.168.12.0
NOTE - The default behavior can be restored by entering the command
no version in the config-router mode.
Router(config)# router rip
Router(config-router)# no version
Verifying & Troubleshooting RIP
show ip route to make sure routers have learned all networks
dynamically
show ip protocols to see information about the routing protocols
used.
debug ip RIP to see live routing updates
CCNA v3.0 Semester 3
You can override the default behavior of RIP by configuring a
particular interface to behave differently.
Overriding Default Behavior of RIP
Interface e0 sends and receives version 1 updates only.
RIP v2 configured on the router.
Router(config)# router rip
Router(config-router)# version 2
Router(config-router)# network 192.168.12.0
Router(config-if)# ip rip send version 1
Router(config-if)# ip rip receive version 1
CCNA v3.0 Semester 3
You can override the default behavior of RIP by configuring a
particular interface to behave differently.
Interface e2 has no special configuration and therefore sends and
receives version 2 by default.
Overriding Default Behavior of RIP
Interface e1 sends and receives both version 1 and 2 updates.
Router(config)# int e1
Router(config-if)# ip rip send version 1 2
Router(config-if)# ip rip receive version 1 2
Router(config)# int e2
CCNA v3.0 Semester 3
CCNA v3.0 Semester 3
Administrative distance of 0 - default
outgoing interface
Administrative distance of 1 - default
Next hop interface
Configuring Static Routes
Remember, an administrator actually enters these routes into the
routing table.
That makes them static route entries – because the router is not
“discovering” those routes.
If for some reason that outgoing interface goes down or is not
available for some reason, then at that time the route will be
removed from the routing table.
Show ip route shows the routing table.
The route would still be in the configuration (because it was
entered globally), but that route could now no longer be used by
the router because the interface it refers to is down for some
reason.
CCNA v3.0 Semester 3
Defaults can always be changed!!!
Just make it higher if you want it to be a “backup” route.
ip route 192.168.2.0 255.255.255.0 192.188.4.1 120
CCNA v3.0 Semester 3
192.168.1.0/24
192.168.3.0/24
192.168.5.0/24
What would you enter to configure a static route from Router C to
the LAN on Router A using outgoing interface?
The LAN on Router B from Router A using next-hop?
CCNA v3.0 Semester 3
The static default route
A router should be configured with a special type of static route –
a default route.
This default route routes packets with destinations that do not
match any of the other routes in the routing table
It is a “gateway of last resort” that allows the router to forward
“destination unknown” packets out a particular interface
ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing
interface]
CCNA v3.0 Semester 3
CCNA v3.0 Semester 3
CCNA v3.0 Semester 3
Classless Routing Protocols
Class B
Class CSmall Networks24 bits8 bits 192 - 2232,097,152254
The Class System
/271/8th of a Class C32
/261/4th of a Class C64
/251/2 of a Class C128
/241 Class C or 1 /24256
/232 Class C or 2 /24s512
/224 Class C or 4 /24s1,024
/218 Class C or 8 /24s2,048
/2016 Class C or 16 /24s4,096
/1932 Class C or 32 /24s8,192
/1864 Class C or 64 /24s16,384
/17128 Class C or 128 /24s32,768
/16256 Class C or 1 Class B65,536
/15512 Class C or 2 Class B131,072
/141,024 Class C or 4 Class B262,144
/132048 Class C or 8 Class B524,288
/124096 Class C or 16 Class B1,048,576
/118192 Class C or 32 Class B2,097,152
/1016384 Class C or 64 Class B4,194,304
/932768 Class C or 128 Class B8,388,608
/865,536 Class C or 256 Class B or 1 Class A16,777,216
Prefix Equivalents