Chapter 6 VLSM and CIDR
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
graziani@cabrillo.edu
Last Updated: 2/24/2009
Note
 My web site is www.cabrillo.edu/~rgraziani.
 For access to these PowerPoint presentations and other
materials, please email me at graziani@cabrillo.edu.
 Additional information is in the Notes section of this PowerPoint.
2
For further information
 This presentation is an
overview of what is
covered in the
curriculum/book.
 For further explanation
and details, please read
the chapter/curriculum.
 Book:
 Routing Protocols
and Concepts
 By Rick Graziani and
Allan Johnson
 ISBN: 1-58713-206-0
 ISBN-13: 978-58713206-3
3
Topics
 Classful and Classless
Addressing
 Classful IP Addressing
 Classful Routing Protocols
 Classless IP Addressing
 Classless Routing Protocols
 VLSM
 VLSM in Action
 VLSM and IP Addresses
 CIDR
 Route Summarization
 Calculating Route
Summarization
4
Classful and Classless
Addressing
 Classful IP Addressing
 Classful Routing Protocols
 Classless IP Addressing
 Classless Routing Protocols
Classful and Classless Routing Protocols
 Routing protocols:
 classful or classless.
 This is a result of the evolution from classful to classless IPv4
addressing.
 As networks began to use classless addressing, classless routing
protocols had to be modified or developed to include the subnet
mask in the routing update.
6
Classful IP Addressing
 As of January 2007, there were over 433 million hosts on the Internet.
 IPv4 32-bit address space would now be exhausted if it were not for?
 VLSM - 1993 (RFC 1519)
 CIDR - 1993 (RFC 1519)
 Network Address Translation (NAT) - 1994 (RFC 1631)
 Private addressing - 1996 (RFC 1918)
7
High-Order Bits?
 Only these three choices - No medium sized networks .
 How did they actually come up with these ranges?
 How can a device such as a router quickly determine the subnet
mask of the IP address?
 By examining the first few bits of the address.
8
Classful Routing Protocol
Classful Routing Protocols
 Is the subnet mask included in the routing update?
 No
 How does the router determine the mask?
 Value of the first octet (first 3 bits of the address) or Interface Mask
9
Classful Routing Protocol
R2 applies s0/0/0’s /24
subnet mask (same
major network)
R1 sends a subnet
address out s0/0/0 (same
major network)
10
Classful Routing Protocol
R2 sends a summarized route
out s0/0/1 (different major
network)
R3 applies the default /16 subnet
mask (different major network)
11
Moving Toward Classless Addressing
 By 1992, IETF had serious concerns about:
 The exponential growth of the Internet and Internet routing
tables.
 Eventual exhaustion of 32-bit IPv4 address space.
 1993, IETF introduced classless interdomain routing (CIDR) (RFC
1517).
 More efficient use of IPv4 address space
 Prefix aggregation, which reduced the size of routing tables
12
11111111.00000000.00000000.00000000 /8 (255.0.0.0)
16,777,216 host addresses
11111111.10000000.00000000.00000000 /9 (255.128.0.0)CLASS A8,388,608 host addresses
11111111.11000000.00000000.00000000 /10 (255.192.0.0)
4,194,304 host addresses
ISPs no longer restricted to
11111111.11100000.00000000.00000000
/11 (255.224.0.0)
2,097,152 host addresses
three classes. Can now
11111111.11110000.00000000.00000000
/12 (255.240.0.0)
1,048,576 host addresses
allocate a large range of
11111111.11111000.00000000.00000000
/13 (255.248.0.0)
524,288 host addresses
network addresses based
11111111.11111100.00000000.00000000
/14 (255.252.0.0)
262,144 host addresses
on customer requirements
11111111.11111110.00000000.00000000
/15 (255.254.0.0) 131,072 host addresses
11111111.11111111.00000000.00000000 /16 (255.255.0.0) 65,536 host addresses
11111111.11111111.10000000.00000000 /17 (255.255.128.0)
32,768 host addresses
CLASS B 16,384 host addresses
11111111.11111111.11000000.00000000 /18 (255.255.192.0)
11111111.11111111.11100000.00000000 /19 (255.255.224.0)
8,192 host addresses
11111111.11111111.11110000.00000000 /20 (255.255.240.0)
4,096 host addresses
11111111.11111111.11111000.00000000 /21 (255.255.248.0)
2,048 host addresses
11111111.11111111.11111100.00000000 /22 (255.255.252.0)
1,024 host addresses
11111111.11111111.11111110.00000000 /23 (255.255.254.0)
512 host addresses
11111111.11111111.11111111.00000000 /24 (255.255.255.0)
256 host addresses
11111111.11111111.11111111.10000000 /25 (255.255.255.128)
128 host addresses
CLASS C
11111111.11111111.11111111.11000000 /26 (255.255.255.192)
64 host addresses
11111111.11111111.11111111.11100000 /27 (255.255.255.224)
32 host addresses
11111111.11111111.11111111.11110000 /28 (255.255.255.240)
16 host addresses
11111111.11111111.11111111.11111000 /29 (255.255.255.248)
8 host addresses
11111111.11111111.11111111.11111100 /30 (255.255.255.252)
4 host addresses
11111111.11111111.11111111.11111110 /31 (255.255.255.254)
2 host addresses
13
11111111.11111111.11111111.11111111 /32 (255.255.255.255)
“Host Route”
CIDR and Route Summarization
 CIDR = Route summarization
 A supernet summarizes multiple network addresses with a mask
less than the classful mask.
14
CIDR and Route Summarization
 192.168.0.0/23, 192.168.2.0/23, 192.168.4.0/22, and 192.168.8.0/21
are all subnets of 192.168.0.0/20
15
CIDR and Route Summarization
 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.
16
Classless Routing Protocol
 Classless routing protocols include the subnet mask with the
network address in their routing updates.
17
Classless
Routing
Protocol
 172.16.0.0/16, 172.17.0.0/16, 172.18.0.0/16, and 172.19.0.0/16
summarized as 172.16.0.0/14.
 What is this called? (Subnet mask is less than the classful default
mask.)
 Supernet
 /14 (255.252.0.0) subnet mask is included in the routing update.
18
VLSM
 VLSM in Action
 VLSM and IP Addresses
VLSM
 The network 10.0.0.0/8 has been subnetted using the subnet mask
of /16, which gives the potential of 256 subnets:
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
.
.
.
10.255.0.0/16
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VLSM
 Any of these /16 subnets can be subnetted further.
 For example the 10.1.0.0/16 subnet is subnetted again using the /24 mask.
21




10.1.0.0/16 subnet is subnetted again using the /24 mask
10.2.0.0/16 subnet is also subnetted again with a /24 mask.
10.3.0.0/16 subnet is subnetted again with the /28 mask.
10.4.0.0/16 subnet is subnetted again with the /20 mask.
22
A 10.1.4.10/24
 Individual host addresses are assigned from the addresses of “subsubnets.”
 What would be a valid Host IP address for Host A?
23
VLSM: A different way to look at it
 Subnet 10.0.0.0/8 into /16 subnets.
 Subnet 10.1.0.0/16 into /24 subnets.
24
VLSM: A different way to look at it
 Subnet 10.2.0.0/16 into /24 subnets.
 Subnets ranging from 10.2.0.0/24 to 10.2.255.0/24
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VLSM: A different way to look at it
 Subnet 10.3.0.0/16 is further subnetted with a /28 mask
 14 host addresses per subnet.
 Subnets ranging from 10.3.0.0/28 to 10.3.255.240/28.
26
VLSM: A different way to look at it
 Subnet 10.4.0.0/16 subnetted with a /20 mask
 4094 host addresses per subnet
 subnets ranging from 10.4.0.0/20 to 10.4.240.0/20
27
VLSM
These subnets could be
subnetted further!
All other /16
subnets are still
available for use
as /16 networks or
to be subnetted.
28
VLSM
What are the valid
host IP Addresses?
Hosts are assigned
an IP address and
mask from a
specific subnet.
10.2.1.55/24
10.2.5.55/24
All other /16
subnets are
still available
for use as
/16 networks
or to be
subnetted.
10.255.0.5/16
10.4.0.55/20
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VLSM
Are these valid host
IP Addresses?
Host can only be a member
of the subnet. Host can NOT
be a member of the network
that was subnetted.
YES!
10.2.1.55/24
10.2.0.55/16
All other /16
subnets are still
available for use
as /16 networks or
to be subnetted.
NO!
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VLSM 1
255.255.255.240 or /28
31
VLSM 2
/30 – Gives 4 addresses
- 2 usable host addresses
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VLSM 2 – Possible
/30 options
Conflicts
Existing /27
Networks
Conflict
Conflict
/30
Choices
128 64 32 16
8
4
2
1
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1
0
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1
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Answer
33
VLSM 2 – Our new VSLM Subnet
128 64 32 16
8
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Existing /27
.64
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Networks
.96
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---------------------------------------------.192
1
1
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.192
.193
1
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1 (1st hst)
Network
.194
1
1
0
0
0
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1
0 (2nd hst)
.195
1
1
0
0
0
0
1
1 (Bcast)
34
VLSM 2 – Other VLSM Subnets
Existing /27
Networks
.192
Network
Other /30
Networks
128 64 32 16
8
4
2
1
.64
0
1
0
0
0
0
0
0
.96
0
1
1
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.128
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--------------------------------------.192
1
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.196
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.200
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.216
1
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.220
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0
35
CIDR
 Route Summarization
 Calculating Route Summarization
CIDR
CIDR Report: www.cidr-report.org
 CIDR allows routing protocols to summarize multiple networks, a
block of addresses, as a single route.
 An example is 172.16.1.0/24.
37
Route Summarization
 Route summarization (route aggregation) - Process of advertising a
contiguous set of addresses as a single address with a lessspecific, shorter subnet mask.
 Remember that CIDR is a form of route summarization and is
synonymous with the term…?
 Supernetting.
38
Route Summarization
 CIDR ignores the limitation of classful boundaries and allows summarization with
masks that are less than that of the default classful mask.
 What type of routing protocols can propagate (distribute) supernets?
 Classless routing protocols
 Why?
 Classless routing protocols include both the network address and the mask in the
routing update.
 Why can’t a classful routing protocol propagate supernets?
 Classful routing protocols cannot include supernets in their routing updates
because they cannot apply a mask less than the default classful mask.
39
Route Summarization
 For example, RIPv1 will summarize 172.30.0.0/24 subnets
(172,30.1.0/24, 172.30.2.0/24 and 172.30.3.0/24) as 172.30.0.0.
 R3 applies the /8 mask (classful routing protocol)
40
Route Summarization
 Why is this static route a supernet?
 The /13 mask is less than the default Class B /16.
41
More specific match?
172.16.0.0/16
S0/0/0
Different example from book.
172.16.10.0/24
S0/0/1
 Could a router have both a specific route entry and a summary route entry
covering the same network.
 Yes
 What if a packet with the destination IP address 172.16.10.10 entered this
router? Where would it be forwarded and why?
 The packet has a more specific (longer) match with 172.16.10.0/24, so
S0/0/1 would be used to forward this packet.
 A minimum of 24 bits match between the IP address and the route.
 What is a packets with the destination IP address 172.16.20.10 entered this
router? Where would it be forwarded and why?
 The packet only has a match with the less specific172.16.10.0/24, so
S0/0/1 would be used to forward this packet
 A minimum of 16 bits match between the IP address and the route.
42
Calculating Route Summarization
 Calculating route summaries and supernets is identical to the
process that you already learned in Chapter 2.
43
Topics
 Classful and Classless
Addressing
 Classful IP Addressing
 Classful Routing Protocols
 Classless IP Addressing
 Classless Routing Protocols
 VLSM
 VLSM in Action
 VLSM and IP Addresses
 CIDR
 Route Summarization
 Calculating Route
Summarization
44
Chapter 6 VLSM and CIDR
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
graziani@cabrillo.edu
Last Updated: 2/24/2008