Quản trị mạng - Chapter 6: Vlsm and cidr
Company XYZ needs to address 400 hosts.
• Its ISP gives them two contiguous Class C addresses:
– 207.21.54.0/24
– 207.21.55.0/24
• Company XYZ can use a prefix of 207.21.54.0 /23 to supernet
these two contiguous networks. (Yielding 510 hosts)
• 207.21.54.0 /23
– 207.21.54.0/24
– 207.21.55.0/24
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Chapter 6: VLSM and CIDR
CCNA Exploration version 4.0
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Objectives
• 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.
1. VLSM is simply subnetting a subnet
2. Subnets can be further subnetted in multiple levels
3. In addition to subnetting, it became possible to
summarize a large collection of classful networks into
an aggregate route, or supernet.
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Classful and Classless Addressing
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• As of January 2007, there are over 433 million hosts on
internet
Classful IP Addressing
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• Initiatives to conserve IPv4 address
space include:
1. VLSM & CIDR notation (1993,
RFC 1519)
2.Network Address Translation
(1994, RFC 1631)
3. Private Addressing (1996, RFC
1918)
Classful IP Addressing
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Classful IP Addressing
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• 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
Classful IP Addressing
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• The IPv4 Classful Addressing Structure (RFC 790)
• An IP address has 2 parts:
– The network portion: the left side of an IP address
– The host portion: the right side of an IP address
Classful IP Addressing
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Classful IP Addressing
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Classful Routing Protocol
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• Recall that classful routing protocols (i.e. RIPv1)
do not send subnet masks in their routing
updates
– Router receives the routing update could
determine the subnet mask simply by
examining the value of the first octet in the
network address, or by applying its ingress
interface mask for subnetted routes.
– The reason is that the Subnet mask is directly
related to the network address
Classful Routing Protocol
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Classful Routing Protocol
• Convergence ?
R1 R2
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Extra: The problems of IP v4
1. Exhaustion of the Class B network address
space. One fundamental cause of this problem is
the lack of a network class of a size that is
appropriate for mid-sized organization.
• Class C is too small, whereas Class B is too
large for most organizations but was the best
fit available for use with subnetting.
2. Growth of routing tables in Internet routers
beyond the ability of current software, hardware,
and people to effectively manage.
3. Eventual exhaustion of the 32-bit IPv4 address
space.
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Extra: The problems of IP v4
• The design and deployment of CIDR
was intended to solve these problems
by providing a mechanism to slow the
growth of global routing tables and to
reduce the rate of consumption of IPv4
address space.
• It did not and does not attempt to solve
the third problem.
•
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Classless IP Addressing
• Classless Inter-domain Routing (CIDR –
RFC 1517). Allows for:
–More efficient use of IPv4 address space
–Prefix aggregation, which reduced the
size of routing tables (route
summarization )
• Requires subnet mask to be included in
routing update because address class is
meaningless
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•CIDR uses Variable Length
Subnet Masks (VLSM) to
allocate IP addresses to
subnets according to individual
need rather than by class.
CIDR and Route Summarization
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• CIDR allowed for prefix aggregation, which
you already know as route summarization.
• A supernet summarizes multiple network
addresses with a mask less than the
classful mask.
• Propagating VLSM and supernet routes
requires a classless routing protocol,
because the subnet mask can no longer be
determined by the value of the first octet.
CIDR and Route Summarization
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CIDR and Route Summarization
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Classless Routing Protocol
• Characteristics of classless routing
protocols:
–Routing updates include the
subnet mask
–Supports VLSM
–Supports Route Summarization
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Classless Routing Protocol
Routing
Protocol
Routing
updates
Include subnet
Mask
Supports
VLSM
Ability to
send
Supernet
routes
Classful No No No
Classless Yes Yes Yes
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Classless Routing Protocol
• Note:
– Classful routing protocols cannot send supernet routes because the
receiving router will apply the default classful to the network
address in the routing update.
– When a supernet route is in a routing table, for example, as a static
route, a classful routing protocol will not include that route in its
updates.
R1(config)# ip route 172.0.0.0 255.0.0.0 loopback1
RT:
S 172.0.0.0/8
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Classful Addressing
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Subnetting
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Classless Addressing
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VLSM
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• 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
• Animation 6.2.1
VLSM in Action
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VLSM in Action
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207.21.24.192/30
207.21.24.196/30 207.21.24.200/30
207.21.24.204/30
207.21.24.208/30 207.21.24.212/30
207.21.24.32/27
207.21.24.64/27
207.21.24.96/27 207.21.24.128/27
207.21.24.160/27 207.21.24.224/27 207.21.24.0/27
207.21.24.216/30
• This network has seven /27 subnets with 30 hosts each AND seven /30
subnets with 2 hosts each (one left over).
• /30 subnets with 2 hosts per subnet do not waste host addresses on
serial networks .
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VLSM and the Routing Table
Routing Table without VLSM
RouterX#show ip route
207.21.24.0/27 is subnetted, 4 subnets
C 207.21.24.0 is directly connected, Serial0
C 207.21.24.32 is directly connected, Serial1
C 207.21.24.64 is directly connected, Serial2
C 207.21.24.96 is directly connected, FastEthernet0
Routing Table with VLSM
RouterX#show ip route
207.21.24.0/24 is variably subnetted, 4 subnets, 2 masks
C 207.21.24.192 /30 is directly connected, Serial0
C 207.21.24.196 /30 is directly connected, Serial1
C 207.21.24.200 /30 is directly connected, Serial2
C 207.21.24.96 /27 is directly connected, FastEthernet0
• Parent Route shows classful mask instead of subnet mask of the child
routes.
• Each Child Routes includes its subnet mask.
Displays one subnet mask for all child routes.
Classful mask is assumed for the parent route.
Each child routes displays its own subnet mask.
Classful mask is included for the parent route.
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• VLSM – the process of
sub-netting 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
VLSM
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VLSM
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All Zeros and All Ones Subnets
Using the All Ones Subnet
• There is no command to enable or disable the use of the all-ones subnet, it is
enabled by default.
• The use of the all-ones subnet has always been explicitly allowed and the use
of subnet zero is explicitly allowed since Cisco IOS version 12.0.
Router(config)#ip subnet-zero
RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is
obsolete! Modern software will be able to utilize all definable networks."
Today, the use of subnet zero and the all-ones subnet is generally accepted
and most vendors support their use, though, on certain networks,
particularly the ones using legacy software, the use of subnet zero and the
all-ones subnet can lead to problems.
CCO: Subnet Zero and the All-Ones Subnet
0080093f18.shtml
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CIDR
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• 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
Route Summarization
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Route Summarization
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Calculating Route Summarization
• 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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Without CIDR, a
router must
maintain
individual
routing table
entries for these
class B
networks.
With CIDR, a
router can
summarize
these routes
into eight
networks by
using a 13-bit
prefix:
172.24.0.0 /13
1. Count the number of left-most matching bits, /13
2. Add all zeros after the last matching bit:
172.24.0.0 = 10101100 00011000 00000000 00000000
Steps:
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CIDR (Classless Inter-Domain Routing)
1.More efficient routing
– reduced routing table size
– reduced routing update traffic
2. A reduced number of CPU cycles when
recalculating a routing table, or when
sorting through the routing table entries to
find a match
3. Reduced router memory requirements
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CIDR (Classless Inter-Domain Routing)
•Route summarization is also
known as:
–Route aggregation
–Supernetting
•Supernetting is essentially the
inverse of subnetting.
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• Company XYZ needs to address 400 hosts.
• Its ISP gives them two contiguous Class C addresses:
– 207.21.54.0/24
– 207.21.55.0/24
• Company XYZ can use a prefix of 207.21.54.0 /23 to supernet
these two contiguous networks. (Yielding 510 hosts)
• 207.21.54.0 /23
– 207.21.54.0/24
– 207.21.55.0/24
23 bits in common
Supernetting Example
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• With the ISP acting as the addressing authority for a CIDR block of
addresses, the ISP’s customer networks, which include XYZ, can be
advertised among Internet routers as a single supernet.
Supernetting Example
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CIDR and the Provider
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200.199.48.0/24 11001000 11000111 001100 00 00000000
200.199.49.0/24 11001000 11000111 001100 01 00000000
200.199.50.0/24 11001000 11000111 001100 10 00000000
200.199.51.0/24 11001000 11000111 001100 11 00000000
200.199.48.0/22 11001000 11000111 001100 00 00000000
200.199.52.0/24 11001000 11000111 001101 00 00000000
200.199.53.0/24 11001000 11000111 001101 01 00000000
200.199.54.0/24 11001000 11000111 001101 10 00000000
200.199.55.0/24 11001000 11000111 001101 11 00000000
200.199.52.0/22 11001000 11000111 001101 00 00000000
200.199.56.0/24 11001000 11000111 00111 000 00000000
200.199.57.0/24 11001000 11000111 00111 001 00000000
..
200.199.63.0/24 11001000 11000111 00111 111 00000000
200.199.56.0/21 11001000 11000111 00111 000 00000000
CIDR and the provider
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• Dynamic routing protocols must send network address
and mask (prefix-length) information in their routing
updates.
• In other words, CIDR requires classless routing protocols
for dynamic routing.
CIDR Restrictions
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Route flapping
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Extra: Configure a route aggregation with RIP version2
S0/0
RTA(config)#router rip
ver 2
no auto-summary
RTA(config)#int s0/0
ip summary-address rip 200.199.48.0 255.255.252.0
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• RTA(config)#router rip
• ver 2
• no auto-summary
• RTA(config)#int s0/0
• ip summary-address rip
200.199.48.0 255.255.252.0
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Labs
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Summary
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