Supernetting/Classless InterDomain Routing
Lecture Organization
 In this lecture, we will discuss two
important IP addressing techniques that
aim to conserve IP address space and
reduce the size of Internet Routing tables.
◦ We will first discuss Supernetting (a technique
now obsolete)
◦ We shall conclude with an elaborate
discussion on CIDR.
SUPERNETTING
Supernetting (RFC 1338)
Class A and B addresses are almost depleted. However,
class C addresses are still available
 But, the size of class block, 256, is too small
◦ Solution: Supernetting
 Supernetting is the concept of taking two or more
numerically contiguous (Class C) network address
blocks and consolidating them into a single, larger
network address.
 Several class C networks are combined to create a supernet.
◦ Logical and physical consolidation of networks.
◦ Supernetting would also let you advertise just one network
block
 Organization can apply for a set of class C address blocks
instead of just one.
 Organization can then use these addresses in one
supernetwork: Physical consolidation of networks

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A Supernetwork
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Assigning Class C addresses under Supernetting
 Random assignment
◦ Routers treat each block separately.
◦ N entries in routing table for N blocks.
◦ All N addresses belong to one organization.
 Assigning on a set of rules
◦ Making a superblock out of all the assigned
blocks.
◦ Routing table then has a single entry.
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Supernetting Rules

Rule #1: The number of blocks (Class C network addresses)
must be a power of 2. (1, 2, 4, 8, 16…)

Rule #2: The blocks must be contiguous (numbering in the third
octet) in the address space. (no gaps between the blocks)

Rule #3: The third byte of the first address of the superblock
must be evenly divisible by the number of blocks.
◦ Example: If number of blocks is N, the third byte must be
divisible by N.

Rule #4: Single interface condition
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Example
A company needs 600 addresses.
Which of the following set of
class C address blocks can be used to form a Supernet
for this company?
a. 198.47.32.0
b. 198.47.32.0
c. 198.47.31.0
d. 198.47.32.0
198.47.33.0
198.47.42.0
198.47.32.0
198.47.33.0
198.47.34.0
198.47.52.0
198.47.33.0
198.47.34.0
198.47.62.0
198.47.52.0
198.47.35.0
a: No, there are only three blocks.
b: No, the blocks are not contiguous.
c: No, 31 in the first block is not divisible by 4.
d:Yes, all three requirements are fulfilled.
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Supernet Mask
 For A, B, C addresses the first address in the block
and the mask define the block (range of
addresses).
 For subnets, the first address in the subnet and
the subnet mask completely define the subblock
(the range of addresses).
 In case of Supernet, the first address in the block
and the Supernet mask completely defines the
block.
 Supernet is the reverse of subnet
◦ Subnet: more 1s than default.
◦ Supernet: Less 1s than default.
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Example
We need to make a supernetwork out of 16 class
C addresses (sometimes called )blocks.
What is the supernet mask?
We have 16 addresses/blocks.
Default subnet mask: 255.255.255.0=
11111111 11111111 11111111 00000000
For 16 addresses we need to change four least significant bits
in the 3rd octet to 0s in the default mask.
So the mask is
11111111 11111111 11110000 00000000
or
255.255.240.0
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Example
A Supernet has a first address of 205.16.32.0 and a Supernet
mask of 255.255.248.0. How many blocks are in this Supernet
and what is the range of addresses?
The Supernet has 21 1s. The default mask has 24 1s.
Since the difference is 3, there are 23 or 8 blocks in this Supernet.
The blocks are 205.16.32.0 to 205.16.39.0. The first address is 205.16.32.0.
The last address is 205.16.39.255.
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Example
An organization requires 2000 addresses to cater to a large
multimedia laboratory.
Suggest a suitable Supernetting scheme to cater to this
requirement.
Step 1: Determine the size of Supernet Block.
Each Class C address provides 256 addresses.
Use eight Class C networks, or CIDR /21, to give us 2,048 possible addresses:
Step 2: Eight Class C addresses provide 2048 addresses:
Actual number of host addresses obtained is 2046 (cannot use the first and the
last address )
Step 3: Select a starting address that fulfils all Supernetting rules:
The starting address chosen is 192.168.16.0 (satisfies the rules of Supernet)
Starting with 192.168.16.0, all "connected" networks must be consecutive in the
numbering of the third octet. Refer to the table in the next slide.
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Supernetting Plan for the organization
Network
Available
Addresses
Usage
Circumstances
192.168.16.0
1-255
First address not
available
192.168.17.0
0-255
All addresses in range
available
192.168.18.0
0-255
All addresses in range
available
192.168.19.0
0-255
All addresses in range
available
192.168.20.0
0-255
All addresses in range
available
192.168.21.0
0-255
All addresses in range
available
192.168.22.0
0-255
All addresses in range
available
192.168.23.0
0-254
Last address not available
Note: A peculiar way of using addresses. Why?
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Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR)
(RFC 1518/RFC 4632)
 A new addressing scheme which allows for more efficient
allocation of IP addresses than the old Class A, B, and C
address scheme.
◦ CIDR and Supernetting standards developed independently- Not
related.
 Used Synonymously with Route Summarization (aggregation)
and Variable Length Subnet Mask (VLSM) but its different
 CIDR itself refers to the administrative assignment of large
address blocks and the related summarized routes for the
purpose of reducing the size of the routing tables.
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CIDR
Need for CIDR
 Running out of IP addresses
 Running out of capacity in the global routing tables
 How Were These Problems Solved?
◦ Restructuring IP address assignments to increase
efficiency
◦ Hierarchical routing aggregation to minimize route
table entries
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CIDR Procedure
 Administrative
 Technical
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CIDR Procedure
CIDR Procedure
Administrative: A hierarchy of ISPs where larger ISPs control
large blocks of addresses and assign smaller blocks to smaller
ISPs and customers.
 The Internet Assigned Numbers Authority (IANA) issues to
regional Internet registries (RIRs) large, short-prefix CIDR
blocks.
 For example, 62.0.0.0/8 (with over sixteen million addresses)
is administered by RIPE NCC, the European RIR.
 The RIRs, each responsible for a single, large, geographic area,
such as Europe or North America, subdivide these blocks
and allocate subnets to local Internet registries (LIRs).
 Similar subdividing may be repeated several times at lower
levels of delegation.
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CIDR Procedure
 Technical: It involves the process of route
summarization/aggregation as well as creating
variable length subnetting blocks.
◦ Route summarization (also called
route aggregation) can reduce the number of
routes that a router must maintain, because it is a
method of representing a series of network numbers
in a single summary address.
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Figure: Hierarchical routing with ISPs
Restructuring IP Address Assignments
Standard subnetting procedure was used by the main ISP to create
smaller blocks.
 To begin with this ISP was assigned a mask of /20
 Furthermore, Instead of being limited to network identifiers (or
"prefixes") of 8, 16 or 24 bits, The ISP used a mask (prefix length )
of its choice. To create sub blocks.
 CIDR currently normally uses prefixes anywhere from 13 to 27
bits.
 Thus, blocks of addresses can be assigned to networks as small as
32 hosts or to those with over 500,000 hosts.
 CIDR address includes the standard 32-bit IP address and also
information on how many bits are used for the network prefix.
For example, in the CIDR address 206.13.01.48/25, the "/25"
indicates the first 25 bits are used to identify the unique network
leaving the remaining bits to identify the specific host.
 Note: n= prefix length, 32-n = suffix length

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Block allocation
CIDR Rules
 Rule #1: Number of addresses in a block
must be a power of 2.
 Rule #2: The beginning address of a block
must be evenly divisible by the number of
addresses.
 Rule #3: Addresses must be contiguous.
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What is a block?
 Block Size: In CIDR notation, the "block"
refers to the number of host addresses
available within a network defined by the
CIDR mask.
 Calculation: The block size is calculated as
2^(32-n) for IPv4, where n is the prefix
length (the number after the slash in CIDR
notation).
 Example: In a /24 network, there are 2^(3224) = 2^8 = 256 total addresses.
 In other words, a /24 network (block)
contains 256 addresses.
CIDR: Rule#2
Rule#3: First IP Address of the
block must be divisible by the
size of the block.

If any binary pattern
consisting of (m + n) bits is
divided by 2n, then• Remainder is least
significant n bits
• Quotient is most
significant m bits

So, any binary pattern is
divisible by 2n, if and only if
its least significant n bits
are 0.
Example
 Consider a binary pattern01100100.00000001.00000010.01000000
(represented as 100.1.2.64)
•
It is divisible by 25 since its least
significant 5 bits are zero.
•
It is divisible by 26 since its least
significant 6 bits are zero.
•
It is not divisible by 27 since its least
significant 7 bits are not zero.
CIDR Rules
Rule 2
Example:
Let's consider a /29 CIDR block, which has 8 addresses (2^3 = 8).
Let's look at two potential starting addresses:
1. Valid starting address: 192.168.1.8
Binary: 11000000.10101000.00000001.00001000
2. Invalid starting address: 192.168.1.10
Binary: 11000000.10101000.00000001.00001010
Analysis: For 192.168.1.8:
Last 3 bits: 000
This address is valid because the last 3 bits are all zeros, making it
divisible by 8.
For 192.168.1.10:
Last 3 bits: 010
This address is invalid because the last 3 bits are not all zeros, making
it not divisible by 8.
Example
 Which of the following can be the
beginning address of a block that contains
16 addresses?
205.16.37.32
190.16.42.44
17.17.33.80
123.45.24.52
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Example

Which of the following can be the beginning
address of a block that contains 16 addresses?


205.16.37.32
190.16.42.44
ANS:
Option 1 fulfills both the requirements and
therefore can serve as the beginning address of
the block.
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Example
 Organization has a block with beginning address
and prefix length: 205.16.37.24/29.
◦ What is the range of addresses contained in the block
(BLOCK SIZE)?
◦ What is the last address in the block?
ANS
 To find the last address: keep first 29 bits and
change the remaining bits to all 1s: 111
 There are only 8 addresses in the block.
 Last address; 205.16.37.31.
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Creating sub-blocks using subnetting
 The number of addresses in the subblock should be a
power of 2.
 The prefix length (mask) for each subnetwork should be
found using the following formula:
nsub= n +log2(N/Nsub)
Where:
nsub= subnetwork mask
n: original mask
N: number of addresses granted to the organization
Nsub: assigned number of addresses to each subnetwork
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Exercise
An organization is granted the block 130.34.12.64/26. The organization
needs four subnetworks, each with an equal number of hosts. Design the
subnetworks and find the information about each network.
Solution
 The number of addresses for the whole network can be found as N = 232
− 26 = 64. The first address in the network is 130.34.12.64/26 and the last
address is 130.34.12.127/26.We now design the subnetworks:
1.
We grant 16 addresses for each subnetwork to meet the first
requirement (64/16 is a power of 2).
2.
The subnetwork mask for each subnetwork is:

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Address Aggregation
 The CIDR addressing scheme also enables "route
aggregation" in which a single high-level route entry can
represent many lower-level routes in the global routing
tables.
 ICANN assigns a block of addresses to an ISP.
 Each ISP in turn, divides its assigned block into smaller
sub blocks and grants the sub-blocks to its customers.
 Many blocks of addresses aggregated in one block and
granted to one ISP.
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Address Aggregation- Example
Top Level Address : 140.24.7.0/24
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Route Aggregation/Summarization: Procedure
 Step 1: Write down the binary version of each
component subnet, one on top of another.
 Step 2: Inspect the binary values to find how many
consecutive bits have the exact same value in all
component subnets. That number of bits is the prefix
length.
 Step 3: Write a new 32-bit number at the bottom of
the list by copying y bits from the previous number, y
being the prefix length. Write binary 0s for remaining
bits.
 Step 4: Convert the bits to decimal.
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Example

An organization is assigned the following 4 sub blocks: 100.16.0.0 /16,
100.17.0.0 /16, 100.18.0.0 /16, and 100.19.0.0 /16. Find the
summarized address.
◦ 100 16 = 01100100 00010000
◦ 100 17 = 01100100 00010001
◦ 100 18 = 01100100 00010010
◦ 100 19 = 01100100 00010011
Interesting Octet: 3rd octet
 Mask: Common bits set to 1: 255.252.0.0
 Number of common bits – mask Length.
 Summarized route: 100.16.0.0/14
 Verification

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Another Example
Summarize the network given below:
172.16.0.0/24
172.16.1.0/24
172.16.2.0/24
172.16.3.0/24
Solution
The binary equivalents of the shown addresses are as follows:
3rd octet is the significant one, let's convert 3rd octet to binary.
172.16.0.0 = 172.16.000000 00.00000000
172.16.1.0 = 172.16.000000 01.00000000
172.16.2.0 = 172.16.000000 10.00000000
172.16.3.0 = 172.16.000000 11.00000000
Here the first 16 bits (172.16) are fixed, and the first 6 bits of the 3rd octet is identical.
Now 22 bits are fixed represents the network portion and the remaining is 12 bits are our
host portion.
So, the summary address is 172.16.0.0/22.
Verification
Another Example
Summarize the network given below:
172.31.20.0/24
172.31.21.0/24
172.31.22.0/24
172.31.23.0/24
172.31.24.0/24
Solution
The binary equivalents of the shown addresses are as follows:
3rd octet is the significant one:
172.31.20.0/24 10101100 00011111 0001 0100 00000000
172.31.21.0/24 10101100 00011111 0001 0101 00000000
172.31.22.0/24 10101100 00011111 0001 0110 00000000
172.31.23.0/24 10101100 00011111 0001 0111 00000000
172.31.24.0/24 10101100 00011111 0001 1011 00000000
172.31.16.0/20: Is this correct??
Previous Example Analysis
 What do you observe?
Two types of summaries:
 Inclusive Summary
 Exclusive Summary
Exercise
Summarize the network given below:
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
192.168.5.0/24
192.168.6.0/24
192.168.7.0/24
How about this crooked block? Can we summarise these addresses in a
meaningful way?
192.168.1.160/30
192.168.1.164/29
192.168.1.172/29
192.168.1.180/30
Forwarding with Classless Addressing
In classful addressing we can
have a routing table with
three columns;
in classless addressing, we
need at least four columns.
Example
Example; Show the forwarding process if the packet arrives at R1 with destination
address 201.4.22.35
Routing table for router R1
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Longest Prefix Matching
Top Level Address : 140.24.7.0/24
What happens if Organization 4 can be no longer connected to R1?
It moves under R4 under a different service Provider which is in
charge of a different address block
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Longest Prefix Matching: Exception Route
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CIDR versus Variable Length Subnet Mask
(VLSM)
 Similarities
◦ Recursive scheme to create smaller subnetworks.
◦ Extended Prefix support
◦ Longest prefix match forwarding algorithm.
◦ Topologically significant addresses for aggregation.
 Differences
◦ Recursion visibility
◦ Classful verses Classless
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CIDR versus Supernetting

Supernetting was not introduced with CIDR.
◦ RFC 1338 (Supernetting) : a standalone strategy for improving the
aggregatability of IP address blocks
◦ Introduced 15 months before CIDR
◦ CIDR made supernetting infinitely more useful.

CIDR: Not a summarization of classful addresses(such as class C in
Supernetting)
◦ Works on variable length blocks called network prefixes that can be
carved out of any type of address space (A, B or C).

When CIDR is deployed, there is no need for Supernetting.

Furthermore, an organization can use subnetting in its allocated block
of addresses. As an example, if the site prefix given to an organization
is /17, the subnet prefix length can be 20 to create 8 subnets. (23) .
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CIDR versus Supernetting

CIDR: An administrative procedure for arranging the Internet
architecture in a hierarchy.
Supernetting is simply a technical procedure for summarizing
contiguous Class C addresses.

Logical versus Physical consolidation.
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Address Aggregation