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Objectives
Compare and contrast classful and classless IPaddressing.
Review VLSM and explain the benefits of classless IPaddressing.
Describe the role of the Classless Inter-DomainRouting (CIDR) standard in making efficient use ofscarce IPv4 addresses
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Introduction
Prior to 1981, IP addresses used only the first 8 bits tospecify the network portion of the address
In 1981, RFC 791 modified the IPv4 32-bit address toallow 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 oninternet
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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The High Order BitsThese are the leftmost bits in a 32 bit address
Classful and Classless IP Addressing
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Classes of IP addresses are identified by the decimalnumber 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 and Classless IP Addressing
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The IPv4 Classful Addressing Structure (RFC 790) An IP address has 2 parts:
The network portion: Found on the left side of an IPaddress
The host portion: Found on the right side of an IPaddress
Classful and Classless IP Addressing
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Classful and Classless IP Addressing
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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
Classful and Classless IP Addressing
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Classless Inter-domain Routing (CIDR RFC 1517)
Allows for:
More efficient use of IPv4 address space.
Route summarization.
Requires subnet mask to be included in routing update
because address class is meaningless.
Classful and Classless IP Addressing
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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
RoutingProtocol
RoutingupdatesInclude
subnetMask
SupportsVLSM
Ability to sendSupernet routes
Classful No No No
Classless Yes Yes Yes
Classless Routing Protocol
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VLSM
Classful routing Only allows for one
subnet mask for allnetworks
VLSM & classless routing This is the process of
subnetting a subnet
More than one subnet
mask can be used.
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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 ANDseven/30 subnets with 2 hosts each (one left over).
/30 subnets with 2 hosts per subnet do not waste host addresses onserial 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 childroutes.
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
VLSM the process ofsub-netting a subnet tofit your needs
- Example:
Subnet 10.1.0.0/16, 8
more bits are borrowedagain, to create 256subnets with a /24 mask.
- Mask allows for 254
host addresses persubnet
- Subnets range from:10.1.0.0 / 24 to
10.1.255.0 / 24
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All Zeros and All Ones Subnets
There is no command to enable or disable the use of the all-ones subnet,
it is enabled by default.
Router(config)#ip subnet-zero
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.
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.
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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 the172.16.0.0 /16 to 172.23.0.0 /16 classful networks
Classless Inter-Domain Routing (CIDR)
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Classless Inter-Domain Routing (CIDR)
Steps to calculate a routesummary List networks in binary
format
Count number of left
most matching bits todetermine summaryroutes mask
Copy the matching bitsand add zero bits to
determine thesummarized networkaddress
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Without CIDR, arouter mustmaintainindividualrouting table
entries for theseclass Bnetworks.
With CIDR, a
router cansummarizethese routesinto eightnetworks byusing 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)
By using a prefix address to summarizes routes,administrators can keep routing table entries manageable,which means the following
More efficient routing
A reduced number of CPU cycles when recalculating arouting table, or when sorting through the routing tableentries to find a match
Reduced router memory requirements
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 supernetthese 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 ofaddresses, the ISPs 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 00000000200.199.49.0/24 11001000 11000111 001100 01 00000000200.199.50.0/24 11001000 11000111 001100 10 00000000200.199.51.0/24 11001000 11000111 001100 11 00000000200.199.48.0/22 11001000 11000111 001100 00 00000000
200.199.52.0/24 11001000 11000111 001101 00 00000000200.199.53.0/24 11001000 11000111 001101 01 00000000200.199.54.0/24 11001000 11000111 001101 10 00000000200.199.55.0/24 11001000 11000111 001101 11 00000000200.199.52.0/22 11001000 11000111 001101 00 00000000
200.199.56.0/24 11001000 11000111 00111 000 00000000200.199.57.0/24 11001000 11000111 00111 001 00000000..
200.199.63.0/24 11001000 11000111 00111 111 00000000200.199.56.0/21 11001000 11000111 00111 000 00000000
CIDR and the provider
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Dynamic routing protocols must send network addressandmask(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
Route flapping occurs when a router interface alternates rapidly between theup and down states.
Route flapping, and it can cripple a router with excessive updates andrecalculations.
However, the summarization configuration prevents the RTC route flappingfrom affecting any other routers.
The loss of one network does not invalidate the route to the supernet. While RTC may be kept busy dealing with its own route flap, RTZ, and all
upstream routers, are unaware of any downstream problem.
Summarization effectively insulates the other routers from the problem of routeflapping.
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Summary
Classful IP addressing
IPv4 addresses have 2 parts:
Network portion found on left side of an IP address
Host portion found on right side 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 InterDomain Routing (CIDR) and Variable Length Subnet Mask (VLSM)are used to try and conserve address space.
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Classful Routing Updates Subnet masks are not sent in routing updates Classless IP addressing
Subnet masks are included in updates
Benefit of classless IP addressing Can create additional network addresses using a
subnet mask that fits your needs.
Saving IP address space.
Uses Classless Interdomain Routing (CIDR).
Summary
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CIDR
Uses IP addresses more efficiently throughuse of VLSM
VLSM is the process of subnetting a subnet.
Allows for route summarization
Route summarization is representingmultiple contiguous routes with a single
route.
Summary
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