1. Introduction to Classless Routing
CCNA v3.0 Module 1
Introduction to Classless Routing

2. What 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).

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.

4. Routing Protocols Supporting VLSM
RIP v2
EIGRP
OSPF

5. Addressing a Network with Standard Subnetting
Site A has two Ethernet networks
Site B had one Ethernet network
Site C had one Ethernet network
/24
How many network addresses are needed?
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
10 users
8 users

6. Addressing a Network with Standard Subnetting
Site A
Site B
Site C
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.

7. 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).
Site A
Site B
Site C
25 users
10 users
8 users

8. 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

9. Provides 1 network with 256 addresses.
Subnetting in a Box
In a class C network there are 256 addresses.
Provides 1 network with 256 addresses.
When we subnet the address, we break it down in to smaller units or subnets.
Subnet mask:
256 addresses
255

10. Subnetting in a Box 128 addresses
Borrowing 1 bit would break the 256 addresses in to two parts (networks)
Providing 2 networks each with 128 addresses.
Subnet mask:
128
127
128 addresses
255

11. Subnetting in a Box Subnet mask: 255.255.255.192. 64 addresses
Borrowing 2 bits would break each of the 2 networks in half again.
Providing 4 networks, each with 64 addresses.
Subnet mask:
128
127
64 addresses
64 addresses 64 63
192
191
255

12. Subnetting in a Box Subnet mask: 255.255.255.224. 32 addresses
Borrowing 3 bits would break each of these 4 networks in half again.
Providing 8 networks, each with 32 addresses.
Subnet mask:
128
127
32 addresses 31 32
32 addresses
159
160 64
192 63
191
32 addresses 95 96
32 addresses
223
224
255

13. Subnetting in a Box Subnet mask: 255.255.255.240. 16 addresses
Borrowing 4 bits would break each of these 8 networks in half again.
Providing 16 networks, each with 16 addresses.
Subnet mask:
128
127 32
160
16 addresses
16 addresses
16 addresses
16 addresses 16 15 48 47
144
143
176
175 64
192 63
191 31
159 96
224
16 addresses
16 addresses
16 addresses
16 addresses 80 79
112
111
208
207
240
239 95
223
255

14. 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.

15. Addressing a Network Using VLSM
To subnet using VLSM, identify the LAN with the largest number of hosts. Subnet the address /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
10 users
8 users

16. Addressing a Network Using VLSM
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.
Site A
Site B
Site C
25 users
10 users
8 users
WAN links
Free Addresses
WAN 1 & 2
Site B & C
Site B
Site C
Site A
Free Addresses

17. 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.
Site A
Site B
Site C
25 users
10 users
8 users
/30
/30
/27
/27
/28
/28

18. Addressing a Network Using VLSM Exercise 1
Your company IP network is /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
Branch 1
60 users
12 users
Branch 2
Branch 3
Branch 4
Branch 5
ISP

19. Subnet according to the largest subnet needed. (Headquarters 60 hosts)
/24
Subnet according to the largest subnet needed. (Headquarters 60 hosts)
128
127 64
192 63
191
Borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.
255

20. Playing it safe, we will not use the first subnet (subnet 0).64
128
192
We will start addressing with /26.
Headquarters needs 60 hosts, so we will assign them
Headquarters
60 hosts
26 bit mask or /26
( )

21. The 5 Branch offices need 12 hosts each.64
128
192
160
Branch 1 12 hosts /28 ( )
Branch 2 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
The next address block available is the block. Use VLSM.
144
176
Headquarters
60 hosts
26 bit mask or /26
( )
Using a /28 mask will give us 16 hosts at each location. This will take care of 4 of the Branch offices.

22. To obtain a block for Branch 5, we will need to subnet the. 192 -
To obtain a block for Branch 5, we will need to subnet the block using a /28 mask. 64
128
192
160
Branch 1 12 hosts /28 ( )
Branch 2 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
144
176
224
Branch 5 12 hosts /28 ( )
Headquarters
60 hosts
26 bit mask or /26
( )
208

23. Now connect the 5 WAN links to the Branch offices.
These are point-to-point connections and only require 2 addresses.
160
128
Branch 1 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
144
176
Branch 2 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
232 64
192
224
WAN 1
WAN 2
WAN 3
WAN 4
Branch 5 12 hosts /28 ( )
Here we will use a /30 mask to further subnet the subnets.
Headquarters
60 hosts
26 bit mask or /26
( )
228
236
240
248
208
WAN 5
244

24. 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.
160
128
Branch 1 12 hosts /28 ( )
Branch 3 12 hosts /28 ( )
144
176
Branch 2 12 hosts /28 ( )
Branch 4 12 hosts /28 ( )
232 64
192
224
WAN 1
WAN 2
WAN 3
WAN 4
Branch 5 12 hosts /28 ( )
Headquarters
60 hosts
26 bit mask or /26
( )
228
236
240
248
208
WAN 5
244

25. Address provided by ISP
Applying the Addresses to the Topology
Address provided by ISP
/26
/28
/28
/28
/28
/28
/30
/30
/30
/30
/30

26. Classful Addressing The IPv4 address architecture uses (a/n) Class A
8 bit network number for Class A addresses
16 bit network number for Class B addresses
24 bit network number for Class C addresses
Class A
Network
Host
Class B
Network
Host 1
Class C
Network
Host 1

27. Classful Addressing
Classful addressing (A, B, C…) is obsolete.

28. Classless Interdomain Routing
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.

29. Classless Interdomain Routing
CIDR sounds a lot like VLSM
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

30. 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.

31. 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

32. 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 ( /24)
1/256th of a Class B ( /24)
1/65,536th of a Class A ( /24)
Using a /24 prefix, each of these specifies eight host bits which will support 254 hosts.

34. Route Aggregation w/ CIDR or (Summarization)
You need 500 addresses.
Given two consecutive /24 addresses:
( /24 and /24)
This address space could be advertised to the rest of the Internet as /23.
Why? (the two /24s have the first 23 bits in common).
23 bits network prefix

35. CIDR Scenario continued
If the ISP owns all of the networks (256 /24s), why should it advertise all of them separately?
Instead, it could simply advertise /16 (which would be /24 through /24).
This would reduce the size of the routing tables on the router to which the routes are advertised.
.0.0
.255.0
16 bits network prefix

36. CIDR Scenario continued
The summary of route /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.).
Examples of starting addresses

37. Network Prefixes
23 bits
/23
/23
/23
/23
/23
/23

38. Network Prefixes
22 bits
/22
/22
/22

39. Network Prefixes
21 bits
/21

40. CIDR in a Nutshell
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).

41. CIDR Example Objective Create an addressing scheme using VLSM.
Scenario
You are assigned the CIDR address /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

42. Dissect the problem Given the CIDR address 200.32.108.0 /22
How many /24 networks do we have?
How many host addresses do we have?
What is the largest LAN requirement?
300 users
100 users

43. Host required - 300, 100, 100, 100, and 3 WAN links
Address given /22
Host required - 300, 100, 100, 100, and 3 WAN links
255
255
255
255

44. Host required - 300, 100, 100, 100, and 3 WAN links
Address given /22
Host required - 300, 100, 100, 100, and 3 WAN links
/23
300 hosts
255
255
255
255

45. Host required - 300, 100, 100, 100, and 3 WAN links
Address given /22
Host required - 300, 100, 100, 100, and 3 WAN links
127
128
/25
100 hosts
/25
/23
300 hosts
255
255
255
255

46. Host required - 300, 100, 100, 100, and 3 WAN links
Address given /22
Host required - 300, 100, 100, 100, and 3 WAN links
128
/25
100 hosts
/25
100 hosts
/23
300 hosts
255
127
255
127
128
/25
100 hosts
255
255

47. Host required - 300, 100, 100, 100, and 3 WAN links
Address given /22
Host required - 300, 100, 100, 100, and 3 WAN links
128
/25
100 hosts
/25
100 hosts
/23
300 hosts
255
127
255
128
/25
100 hosts
191
192
223
224
240
239
248
247
WAN links /30
243
252
251
244
255
127
255

48. CIDR Result Given the CIDR address 200.32.108.0 /22 200.32.108.0 /23
300 users
100 users
/23
/25
/25
/25
/30
/30
/30
Two /24s

49. 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

50. Routing Information Protocol (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.

51. RIP Version 2 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.

52. RIP V2 is RIP V1 with extensions
Subnet masks carried with each route entry
Authentication of routing updates
Next-hop addresses carried with each route entry
External route tags
Multicast route updates

53. 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

54. 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

55. 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

56. RIP v1 & RIP v2 comparisons
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

57. 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
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.

58. 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
NOTE - The default behavior can be restored by entering the command no version in the config-router mode.
Router(config-router)# no version

59. 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

60. Overriding Default Behavior of RIP
You can override the default behavior of RIP by configuring a particular interface to behave differently.
Router(config)# router rip Router(config-router)# version 2 Router(config-router)# network Router(config-router)# exit
RIP v2 configured on the router.
Router(config)# int e0 Router(config-if)# ip address Router(config-if)# ip rip send version 1 Router(config-if)# ip rip receive version 1
Interface e0 sends and receives version 1 updates only.

61. Overriding Default Behavior of RIP
You can override the default behavior of RIP by configuring a particular interface to behave differently.
Router(config)# int e1 Router(config-if)# ip address Router(config-if)# ip rip send version 1 2 Router(config-if)# ip rip receive version 1 2
Interface e1 sends and receives both version 1 and 2 updates.
Interface e2 has no special configuration and therefore sends and receives version 2 by default.
Router(config)# int e2 Router(config-if)# ip address

62. Review of Static & Default Routing

63. Configuring static routes w/ outgoing interface
Administrative distance of 0 - default

64. Configuring static routes w/ next-hop IP address
Next hop interface
Administrative distance of 1 - default

65. 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.

66. Administrative Distance
What is the default for a outgoing interface?
What is the default for the next-hop address?
Defaults can always be changed!!!
Just make it higher if you want it to be a “backup” route.
ip route

67. The LAN on Router B from Router A using next-hop?
Router C
S /24
S /24
/24
/24
/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?

68. 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 [next-hop-address | outgoing interface]

69. Default Route on non-directly connected networks

70. Default Route on non-directly connected networks

71. Introduction to Classless Routing
CCNA v3.0 Module 1
Introduction to Classless Routing