Subnetting & CIDR

cursefarmNetworking and Communications

Oct 24, 2013 (4 years and 16 days ago)

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Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Subnetting & CIDR

Tahir Azim


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Announcements


Participate in NASCON, FAST
-
NU Islamabad


Assignment 1 deadline extended to Tuesday
due to no BIT
-
7 classes on Monday


From last time:


Packet bursting
: An approach to increasing the speed
of
802.11g
-
based wireless networks by unwrapping
short
802.11g

packets and rebundling them into a
larger
packet

to reduce the impact of mandatory gaps
between packets (jwire.com)


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Subnetting


Subnetting is a form of hierarchical routing.


Subnets are usually represented via an address
plus a subnet mask or “netmask”.


e.g.


nickm@elaine17.Stanford.EDU > ifconfig hme0


hme0: flags=863<UP,BROADCAST,NOTRAILERS,RUNNING,MULTICAST>
mtu 1500


inet 171.64.15.82 netmask ffffff00 broadcast 171.64.15.255



Netmask ffffff00: the first 24 bits are the subnet
ID, and the last 8 bits are the host ID.


Can also be represented by a “prefix + length”,
e.g. 171.64.15.0/24, or just 171.64.15/24.


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Subnetting

CLASS “B”

e.g. Company


10

Net ID

Host
-
ID

2

14

16


10

Net ID

Host
-
ID

2

14

16

0000

Subnet ID (20)

Subnet

Host ID (12)


10

Net ID

Host
-
ID

2

14

16

1111

Subnet ID (20)

Subnet

Host ID (12)


10

Net ID

Host
-
ID

2

14

16

000000

Subnet ID (22)

Subnet

Host ID (10)


10

Net ID

Host
-
ID

2

14

16

1111011011

Subnet ID (26)

Subnet

Host ID (6)

e.g. Site

e.g. Dept


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Routing in the presence of subnets


The rest of the
Internet is not aware
of subnets within a
network


Levels: site, subnet,
host


Routing now involves
delivery to the site,
then the subnet and
finally the host


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Example

of subnetting

Gates
-
rtr

171.64.74.58

171.64.74.0/24

171.64.1.178

EndHost

border2
-
rtr

hpr1
-
rtr

bbr2
-
rtr

171.64.1.161

171.64.1.160/27

171.64.0.0/16

AS 32

Class B

Address

171.64.74.1

171.64.1.131

To: cenic.net

To: cogentco.com

171.64.1.152

171.64.1.148

171.64.1.133

171.64.1.144/28

171.64.1.132/30


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Classless Interdomain Routing (CIDR)

Addressing


The IP address space is broken into line segments, or blocks


e.g. Block of 2 addresses, block of 128 addresses etc.


Each block is described by a
prefix
.


A prefix is of the form
x/y

where
x

indicates the prefix of all addresses
in the block, and
y

indicates the length of the prefix.


e.g. The prefix 128.9/16 represents the block containing addresses in
the range: 128.9.0.0 …

128.9.255.255
.


0

2
32
-
1

128.9/16

128.9.0.0

2
16

142.12/19

65/8

128.9.16.14


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Classless Interdomain Routing (CIDR)

Addressing

0

2
32
-
1

128.9/16

128.9.16.14

128.9.16/20

128.9.176/20

128.9.19/24

128.9.25/24

Most specific route = “longest matching prefix”


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Classless Interdomain Routing (CIDR)

Addressing

Prefix aggregation:


If a service provider serves two organizations with
prefixes, it can (sometimes) aggregate them to form a
shorter prefix. Other routers can refer to this shorter
prefix, and so reduce the size of their address table.


E.g. ISP serves 128.9.14.0/24 and 128.9.15.0/24, it
can tell other routers to send it all packets belonging
to the prefix 128.9.14.0/23.

ISP Choice:


In principle, an organization can keep its prefix if it
changes service providers.



Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Size of the Routing Table at
the core of the Internet

Source: http://www.cidr
-
report.org/


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Prefix Length Distribution

Source: Geoff Huston, Jan 2006


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Examples


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Finding the first address


What

is

the

first

address

in

the

block

if

one

of

the

addresses

is

167
.
199
.
170
.
82
/
27
?



Solution
:

The

prefix

length

is

27
,

which

means

that

we

must

keep

the

first

27

bits

as

is

and

change

the

remaining

bits

(
5
)

to

0
s
.

The

following

shows

the

process
:


Address

in

binary
:

10100111

11000111

10101010

01010010


Keep

the

left

27

bits
:

10100111

11000111

10101010

010
00000


Result

in

CIDR

notation
:

167
.
199
.
170
.
64
/
27


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Finding the first address


What is the first address in the block if one of the
addresses is 140.120.84.24/20?


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Finding the last address in the
block


To the first address, add the number of
addresses, minus one


OR


Set all bits that are not part of the CIDR
prefix to 1



Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Example


Find the number of addresses in the block
if one of the addresses is
140.120.84.24/20.


Solution
:

The

prefix

length

is

20
.

The

number

of

addresses

in

the

block

is

2
32

20

or

2
12

or

4096
.

Note

that

this

is

a

large

block

with

4096

addresses
.


Courtesy Nick McKeown (Stanford), Umar Kalim
(NIIT)

Example 2


Find the last address in the block if one of the
addresses is 140.120.84.24/20.



Solution


We found in the previous examples that the first
address is 140.120.80.0/20 and the number of
addresses is 4096. To find the last address, we need
to add 4095 (4096 − 1) to the first address.


Or, set all bits that are not part of the CIDR prefix to 1


140.120.(0101 1111)
2.
(1111 1111)
2
= 140.120.95.255