CCNA 3/Module 1
Introduction to
Classless Routing
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Overview: Classful/Classless Routing
• Classful routing - a network must use the same subnet mask
for the entire network
Network IP
192.168.187.0
Network Subnet Mask
255.255.255.0
Classless routing – using more than one subnet mask for a
network address
• “subnetting a subnet”
Network IP
192.168.187.0
Network Subnet Masks
255.255.255.252
255.255.255.0
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Overview: (Classful) IPv4 Addressing Limits
• IPv4 – 20 years old
• IPv4 – even with subnetting, couldn’t handle the global demand
for Internet connectivity
• Class B space was on the verge of depletion.
• Rapid and substantial increase in the size of the Internet's
routing tables.
• As more Class C's came online, the flood of new network
information threatened Internet routers' capability to
cope.
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Overview: (Classful) IPv4 Addressing Limits
• Provides IP scheme with limitations:
• Class A – 126 networks: 16,777,214 hosts each
• Class B – 65,000 networks: 65,534 hosts each
• Class C – 2 million networks: 254 hosts each
• While available addresses were running out, only 3%
of assigned addresses were
actually being used!
• Subnet zero, broadcast addresses,
pool of unused addresses at
Class A and B sites, etc.
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Overview: Scalability & Routing Tables
• Maximum theoretical routing table size is 60,000 entries.
• Classful addressing would have hit this capacity by
mid-1994.
• Internet growth would have ended.
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1.1.1 What is VLSM and why is it used?
• The purpose of VLSM is to alleviate the shortage of IP addresses
• VLSM allows:
• More than one subnet mask within the same network
• Or . . . Multiple SNMasks with ONE IP Address
• Use of long mask on networks with few hosts
• Use of short mask on networks with many hosts
• In order to use VLSM, the routing protocol must support it.
• Cisco routers with the following routing protocols support VLSM:
• OSPF (Open Shortest Path First)
• IS-IS (Integrated Intermediate System to Intermediate System)
• EIGRP (Enhanced Interior Gateway Routing Protocol)
• RIP v2
• Static Routing
No RIP v1
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1.1.1 What is VLSM and why is it used?
Classful routing protocols use one subnet mask for a single network
• Ex: 192.168.187.0, must use subnet mask 255.255.255.0
VLSM allows a single autonomous system to have networks with
different subnet masks, for example:
• Use a 30-bit subnet mask on network connections
• (255.255.255.252)
• Use a 24-bit subnet mask for user networks up to 250 users
• (255.255.255.0)
• Use a 22-bit subnet mask for user networks up to 1000 users
• (255.255.252.0)
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1.1.2 A waste of space
• In classless routing, it was recommended that first and last
subnet not be used
• First (SN 0) had same address for the network and subnet
• Last subnet (all-1’s) was the broadcast
• Always could have been used, was not recommended
practice
• Address depletion has lead to use of these subnets
• Now acceptable practice to use the first and last subnets in
conjunction with VLSM
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1.1.2 A waste of space
Network Address
192.168.187.0
Borrow 3 bits = SNM
255.255.255.224
Subnets =
0, 32, 64, 96, 128, 160, 192, 224
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1.1.2 A waste of space
Network Address
192.168.187.0
Borrow 3 bits = SNM
255.255.255.224
Subnets =
0, 32, 64, 96, 128, 160, 192, 224
If subnet zero is used, there are 8 useable subnets
• Each subnet can support 30 hosts
• Cisco routers use subnet zero by default IOS v. 12.0+
If no ip subnet-zero command is used on the router, there are 7
useable subnets with 30 hosts per subnet
• If supporting 4 routers (1 subnet each) that need 3 WAN
links to each other, all subnets are used
• No room for growth
• Waste of 28 host addresses for each WAN (point-topoint) links or 1/3 of potential address space
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1.1.2 A waste of space
FOSTER(config)#no ip subnet-zero
• Disables the capability to use subnets that
include the network address of the
unsubnetted network
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1.1.3 When to use VLSM
Design addressing scheme that
allows:
• Growth
• Doesn’t waste addresses on
point-to-point links
VLSM addressing applied instead results in:
•Variable sized subnets
•Take 1 of the 3 subnets and subnet it again
•Example 192.168.187.224 (last subnet)
•Apply a 30 bit mask (225.225.225.252)
•Creates a possible 8 ranges of addresses with 30 bits
•Best solution for point-to-point links – use 2 host addresses
instead of 30
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1.1.4 Calculating subnets with VLSM
VLSM helps to manage IP addresses
• VLSM can use one SNM for a point-to-point link and
one SNM for a LAN
0
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1.1.4 Calculating subnets with VLSM
Foster’s Fabulous Films
•
•
2 routers
• 1 in Hollywood (100 hosts)
• 1 in Ravenna (50 hosts)
• 1 WAN link (2 needed)
IP/NW Address: 192.16.10.0
• Class C
Use the BIGGEST first:
100
50
2
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1.1.4 Calculating subnets with VLSM
Foster’s Fabulous Films
•
•
2 routers
• 1 in Hollywood (100 hosts)
• 1 in Ravenna (50 hosts)
• 1 WAN link (2 needed)
IP/NW Address: 192.16.10.0
• Class C
Use the BIGGEST first:
100 /25
126 usable hosts
50 /26
62 usable hosts
2 /30
2 usable hosts
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1.1.4 Calculating subnets with VLSM
If VLSM were used instead of classful routing:
• A 24-bit mask could be used for LAN segments for 250
hosts
• A 30-bit mask could be used for WAN segments for 2 hosts
• 172.16.32.0/20 (would accommodate 4094 hosts)
• Binary = 10101100.00010000.00100000.00000000
• SNM = 11111111.11111111.11110000.00000000
• VLSM address172.16.32.0/26 (needed for 62 hosts)
• Binary = 10101100.00010000.00100000.00000000
• SNM = 11111111.11111111.11111111.11000000
• If 172.16.32.0/20 used, but only 10 hosts on segment, would
provide 4094 hosts and waste 4084 addresses
• By further subnetting /20 to /26, gain 64 subnets (26) each
supporting 62 hosts
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1.1.4 Calculating Subnets w/VLSM
Procedure to subnet a subnet /20 to /26 using VLSM:
1. Write 172.16.32.0 in binary form
• Binary = 10101100.00010000.00100000.00000000
2. Draw a vertical line between the 20th and 21st bits (the original
subnet boundary)
3. Draw a vertical line between the 26th and 27th bits extending the bits
to segment/host needs
4. Calculate the number of subnet addresses between the two vertical
lines (lowest to highest) in value
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1.1.4 Calculating Subnets w/VLSM
•
Keep in mind that only unused subnets can be further
subnetted
• If any address for a subnet is used cannot be further
subnetted
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1.1.5 Route Aggregation w/VLSM
•
•
•
•
Every network needs a separate entry in routing table
Each subnet needs a separate entry
Aggregation will reduce routing table size
When using VLSM keep subnetwork numbers grouped together
in the network to allow for aggregation by using Classless
InterDomain Routing (CIDR)
• 172.16.14.0
• 172.16.15.0
• Router needs to carry only one route 172.16.14.0/23
14 in Binary = 00001110
15 in Binary = 00001111
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1.1.5 Route Aggregation w/VLSM
• Using CIDR and VLSM prevents address waste and promotes route
aggregation or summarization
• Without summarization, Internet would collapse
• Summarization reduces burden on upstream routers
• This process of summarization continues until entire network is
advertised as a single aggregate route
• Summarization is also called supernetting
• Possible if the routers of a network run a classless routing
protocol such as OSPF or EIGRP
• Consists of IP address and bit mask in routing updates
• The summary route uses prefix common to all addresses of
organization
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1.1.5 Route Aggregation w/VLSM
Carefully assign addresses in a hierarchical fashion to share same
high-order bits for summarization
• A router must know subnets attached in detail
• A router does not need to tell other routers about subnets
• A router using aggregate routes has fewer entries in routing
table
• VLSM allows for summarization of routes
• Works even if networks are not contiguous
• VLSM increases flexibly by summarization on higher-order bits
• Used to calculate the network number of the summary route
• Uses only shared highest-order bits
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1.1.6 Configuring VLSM
• If VLSM is chosen, it must be configured correctly
• Example: 192.168.10.0 (Class C)
• One router has to support 60 hosts, needs 6 bits in host
portion of address to provide 62 possible address
• (26 = 64 – 2 = 62)
192.168.10.0/26 (leaves 6 bits for hosts)
• One router has to support 28 hosts, needs 5 bits in host
portion of address to provide 30 possible hosts
• (25 = 32 – 2 = 30) 192.168.10.64/27 (leaves 5 bits for hosts)
• Two routers have to support 12 hosts each, needs 4 bits in
host portion of address to provide 14 possible hosts
(24 = 16 – 2 = 14) 192.168.10.96/28 (leaves 4 bits for hosts)
192.168.10.112/28 (leaves 4 bits for hosts)
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1.1.6 Configuring VLSM
• Point-to-point connections are:
• 192.168.10.128/30 (2 address required, 2 bits = 2 host addresses)
• 192.168.10.132/30 (2 address required, 2 bits = 2 host addresses)
• 192.168.10.136/30 (2 address required, 2 bits = 2 host addresses)
• Choices = .136 .137 .138 .139
• Configuration as follows for the 192.168.10.136/30 network (.136/30 network address;.139/30 - broadcast address; .137/30 and 138/30 – host
addresses:
• (config)#interface serial 0
• (config-if)#ip address 192.168.10.137 255.255.255.252
• (config)#interface serial1
• (config-if)#ip address 192.168.10.138 255.255.255.252
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1.2.1 RIP History
Internet is a collection of autonomous systems (AS)
• Each AS is administered by a single entity
• Each AS has its own routing technology
Routing protocol used within AS is Interior Gateway Protocol
Routing protocol used between Autonomous Systems is an Exterior Gateway
Protocol
RIP v1:
• is an IGP that is classful
• was designed to work within moderate-sized AS
• is a distance vector routing protocol
• by default, broadcasts entire routing table every 30 seconds
• uses hop count as metric (16 max)
• is capable of load balancing 6 equal-cost paths (4 default)
• Does not send subnet mask information in its updates
• Is not able to support VLSM or CIDR
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1.2.1 RIP History
If the router receives information about a network, and the receiving
interface belongs to same network but is on a different subnet, the
router applies the one subnet mask configured on the receiving
interface
• Class A default classful mask is 255.0.0.0
• Class B default classful mask is 255.255.0.0
• Class C default classful mask is 255.255.255.0
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1.2.2 RIP v2 Features
RIP v2 is an Improved version of RIP v1 with following features:
• Distance vector protocol
• Uses hop count as metric
• Uses hold-down timers (prevent routing loops), default 180 sec.
• Uses split horizon to prevent routing loops
• Uses 16 hops as infinite distance
• Provides prefix routing (sends subnet mask with route update)
• Supports use of classless routing (VLSM)
• Multicasts updates using 224.0.0.9 address for better efficiency
• Provides authentication in updates
• Clear text - default
• MD5 encryption – typically used to encrypt enable secret
passwords (Message-Digest 5)
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1.2.3 Comparing RIP v1 & v2
RIP v1
RIP v2
Easy to configure
Easy to configure
Supports classful routing
Supports classless routing
No subnet info sent with routing
updates (considered a limitation of v1)
Sends subnet mask with routing
update
No authentication
Provides for authentication
Uses hop count
Uses hop count
16 hops as metric for infinite distance 16 hops as metric for infinite distance
Broadcasts routing table updates
255.255.255.255
Multicasts updates 224.0.0.9
Does not support prefix routing (all
devices in same network must use
same subnet mask)
Supports prefix routing (VLSM,
different subnet masks can be used
in same network)
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1.2.4 Configuring RIP v2
To enable a dynamic routing protocol:
1. Select routing protocol
• FOSTER(config)#router rip
• FOSTER(config-router)#version 2
2. Configure routing protocol with the network IP address (identify
physically connected network that will receive routing tables)
• FOSTER(config-router)#network 10.0.0.0
• FOSTER(config-router)#network 172.16.0.0
3. Assign IP/SNM to interfaces
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1.2.5 Verifying RIP v2
FOSTER#show ip protocols
•Shows protocol name
•Tells when updates are sent and
when the next is due
FOSTER#show ip route
•Tells if routers have learned about
a newly added network
•Displays IP routing table
FOSTER#show ip interface brief
•Summary of information
•status of interface
FOSTER#show running-config
Checks for a misconfigured routing
protocol
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1.2.5 Verifying RIP v2
• RIP updates table every 30 seconds
• If no update received in 180 seconds, route marked as down
• If no update after 240 seconds, removes from routing table entry
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1.2.6 Troubleshooting RIP v2
Foster#debug ip rip Displays RIP routing updates as
they are sent and received
Foster#no debug all Turns off all debugging
Foster#undebug all
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1.2.7 Default Routes
Three ways a router learns about paths:
1. Static routes – manual configuration of routes (next hop)
• Uses ip route command
2. Default routes – manually defined path to take when there is no
known route to a destination
3. Dynamic routes – routers lean paths by receiving updates from
other routers
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1.2.7 Default Routes
Static Route Command:
FOSTER(config)# ip route 172.16.1.0 255.255.255.0
172.16.2.1
Default NW
Next hop router
Tells that 8 bits of
subnetting in effect
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1.2.7 Default Routes
DYNAMIC PROTOCOL Default Route Command
FOSTER(config)# ip default-network 192.168.20.0
Default NW
Used to:
1. Give packets that are not ID’d in the routing table a place to go
• Usually a router that connects to the Internet
2. Connect a router with a static default route
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