Download VLSM and CIDR

Routing Protocols
and Concepts
VLSM and CIDR
Chapter 6
Modified by Pete Brierley
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What will we Learn from chapter 6?

Compare and contrast classful and classless IP
addressing.

Review VLSM and explain the benefits of classless IP
addressing.

Describe the role of the Classless Inter-Domain
Routing (CIDR) standard in making efficient use of
scarce IPv4 addresses
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Introduction
 Prior to 1981, IP addresses used only the first 8 bits to
specify the network portion of the address
 In 1981, RFC 791 modified the IPv4 32-bit address to
allow for three different classes
 IP address space was depleting rapidly
the Internet Engineering Task Force (IETF)
introduced Classless Inter-Domain Routing (CIDR)
–CIDR uses Variable Length Subnet Masking
(VLSM) to help conserve address space.
-VLSM is simply subnetting a subnet
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Classful and Classless IP Addressing
 Classful IP addressing
 As of January 2007, there are over 433 million hosts on
internet
 Initiatives to conserve IPv4 address space include:
-VLSM & CIDR notation (1993, RFC 1519)
-Network Address Translation (1994, RFC 1631)
-Private Addressing (1996, RFC 1918)
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Classful and Classless IP Addressing
 The High Order Bits
These are the leftmost bits in a 32 bit address
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Classful and Classless IP Addressing
 Classes of IP addresses are identified by the decimal number of
the 1st octet
Class A address begin with a 0 bit
Range of class A addresses = 0.0.0.0 to 127.255.255.255
Class B address begin with a 1 bit and a 0 bit
Range of class B addresses = 128.0.0.0 to 191.255.255.255
Class C addresses begin with two 1 bits & a 0 bit
Range of class C addresses = 192.0.0.0 to 223.255.255.255.
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Classful and Classless IP Addressing
 The IPv4 Classful Addressing Structure (RFC 790)
An IP address has 2 parts:
-The network portion
Found on the left side of an IP address
-The host portion
Found on the right side of an IP address
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Classful and Classless IP Addressing
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Classful and Classless IP Addressing
 Purpose of a subnet mask
It is used to determine the network portion of an IP
address
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Classful and Classless IP Addressing
 Classful Routing Updates
-Recall that classful routing protocols (i.e. RIPv1)
do not send subnet masks in their routing updates
The reason is that the Subnet mask is
directly related to the network address
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Classful and Classless IP Addressing
 Classless Inter-domain Routing (CIDR – RFC 1517)
Advantage of CIDR :
-More efficient use of IPv4 address
space
-Route summarization
Requires subnet mask to be included in routing update
because address class is meaningless
Recall purpose of a subnet mask:
-To determine the network and host portion
of an IP address
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Classful and Classless IP Addressing
 Classless IP Addressing
 CIDR & Route Summarization
-Variable Length Subnet Masking (VLSM)
-Allows a subnet to be further sub-netted
according to individual needs
-Prefix Aggregation a.k.a. Route Summarization
-CIDR allows for routes to be summarized as a
single route
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Classful and Classless IP Addressing
 Classless Routing Protocol
 Characteristics of classless routing protocols:
-Routing updates include the subnet mask
-Supports VLSM
Supports Route Summarization
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Classful and Classless IP Addressing
 Classless Routing Protocol
Routing
Protocol
Routing
updates
Include
subnet
Mask
Supports Ability to send
VLSM Supernet routes
Classful
No
No
No
Classless
Yes
Yes
Yes
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VLSM
 Classful routing
-only allows for one
subnet mask for all
networks
 VLSM & classless routing
-This is the process
of subnetting a subnet
-More than one
subnet mask can be
used
-More efficient use of IP
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addresses as compared
to classful IP
addressing
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VLSM
 VLSM – the process of subnetting a subnet to fit your
needs
-Example:
Subnet 10.1.0.0/16, 8
more bits are borrowed
again, to create 256
subnets with a /24 mask.
-Mask allows for 254 host
addresses per subnet
-Subnets range from:
10.1.0.0 / 24 to 10.1.255.0 / 24
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Classless Inter-Domain Routing (CIDR)
 Route summarization done by CIDR
-Routes are summarized with masks that are less
than that of the default classful mask
-Example:
172.16.0.0 / 13 is the summarized
route for the 172.16.0.0 / 16 to
172.23.0.0 / 16 classful networks
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Classless Inter-Domain Routing (CIDR)
 Steps to calculate a route
summary
-List networks in binary
format
-Count number of left
most matching bits to
determine summary
route’s mask
-Copy the matching
bits and add zero bits
to determine the
summarized
network address
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CCNA 3/Module 1
An 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”
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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 mid1994.
Internet growth would have ended.
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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 NW
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)
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IS-IS (Integrated Intermediate System to Intermediate System)
EIGRP (Enhanced Interior Gateway Routing Protocol)
RIP v2
Static Routing
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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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A waste of space
 In classless routing, recommended that first and last
subnets have special use; not be used for host addresses
First (SN 0) had same address for the network and subnet
Last subnet (all-1’s) was the broadcast
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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A waste of space
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Network Address
192.168.187.0
Borrow 3 bits = SNM
255.255.255.224
Subnets = (2^H)
0, 32, 64, 96, 128, 160, 192, 224
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A waste of space
Network Address
192.168.187.0
Borrow 3 bits = SNM
255.255.255.224
Subnets = (2^H)
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
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Waste of 28 host addresses for each WAN (point-to-point)
links or 1/3 of potential address space
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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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When to use VLSM
Networking design addressing scheme that allows:
 Growth
 Doesn’t waste addresses on point-to-point links
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When to use VLSM
 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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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
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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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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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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
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4. Calculate the number of subnet addresses between the two vertical lines
(lowest to highest) in value
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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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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 hold only one route 172.16.14.0/23
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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 only if the routers of a network run a classless routing protocol
such as OSPF or EIGRP
•IP address and bit mask included in routing updates
•The summary route uses a prefix common to all addresses of
an organizational group
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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
•
Does not need to tell other routers about subnets
•
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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Configuring VLSM
If VLSM is chosen, it must be configured correctly
Example: 192.168.10.0
One router has to support 60 hosts, needs 6 bits in host portion of
address to provide 62 possible address
(26 = 64 – 2 = 60)
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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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 of the 192.168.10.136/30 subnet
(.136/30 - network address; .137/30 and 138/30 – host addresses .139/30 broadcast address; :
(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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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
• 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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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 or /8
Class B default classful mask is 255.255.0.0 or /16
Class C default classful mask is 255.255.255.0 or /24
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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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Comparing RIP v1 & v2
RIP v1
RIP v2
Easy to configure
Easy to configure
Supports classful routing
Supports classless routing
No subnet mask 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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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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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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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
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Foster# debug ip rip
Displays RIP routing
updates as they are sent
and received
Foster# no debug all
Foster# u all (undebug all)
Turns off all debugging
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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
Default Route Command:
FOSTER(config)# ip route 172.16.1.0 255.255.255.0
Next hop router
Default NW
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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 in the routing table a place to
go typically a router that connects to the Internet
2. Connect a router with a static default route
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What did I Learn from chapter 6?
 Classful IP addressing
IPv4 addresses have 2 parts:
-Network portion found on left portion of an IP address
-Host portion found on right portion of an IP address
Class A, B, & C addresses were designed to provide IP
addresses for different sized organizations
The class of an IP address is determined by the decimal value
found in the 1st octet
IP addresses are running out so the use of Classless Inter
Domain Routing (CIDR) and Variable Length Subnet Mask
(VLSM) are used to conserve address space
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What did I Learn from chapter 6? (con’t)
 Classful Routing Updates
–Subnet masks are not sent in routing updates
 Classless IP addressing
–Benefit of classless IP addressing
Can create additional network
addresses using a subnet mask
that fits your needs
–Uses Classless Interdomain Routing (CIDR)
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What did I Learn from chapter 6? (con’t)
 CIDR
 Uses IP addresses more efficiently through
use of VLSM
-VLSM is the process of
subnetting a subnet
 Allows for route summarization
-Route summarization is
representing multiple contiguous
routes with a single route
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What did I Learn from chapter 6? (con’t)
 Classless Routing Updates
Subnet masks are included in updates
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VLSM & CIDR
Next
RIP Ver2
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