Topics. Subnetting. The Basics of Subnetting Subnet Mask Computing subnets and hosts Subnet Routing Creating a Subnet Example of Subnetting 1/37

(1)

Applied Network Research Group Department of Computer Engineering, Kasetsart University

Subnetting

Surasak Sanguanpong [email protected] http://www.cpe.ku.ac.th/~nguan

Last updated: 27 June 2002

2/37

Topics

z

The Basics of Subnetting

z

Subnet Mask

z

Computing subnets and hosts

z

Subnet Routing

z

Creating a Subnet

(2)

Addressing without Subnets

z

A class B “Flat Network”, more than

65000 hosts

z

How to manage?

z

Performance?

172.16.1.2 172.16.1.3 172.16.2.1 172.16.254.254

172.16.0.0

4/37

Addressing with Subnets

172.16.1.2

172.16.3.3 172.16.3.2

172.16.1.3

z

A class B “subdivided network”, smaller groups

with routers

172.16.2.2 172.16.2.3 172.16.4.2 172.16.4.3

172.16.1.0

172.16.3.0

172.16.2.0

172.16.4.0

(3)

Subnetwork

Subnetwork benefits

Subdivide on IP network number

is an important initial task of

network managers

Increase the network

manager's control over

the address space

Smaller networks

are easier to manage

and troubleshoot

Overall traffic is

reduced, performance

may improve

6/37

Subnet Address

z

A subnet address is created by borrowing bit

from the Host ID and designated it as a Subnet

ID field

Network ID

Subnet ID

Host ID

Network ID

Host ID

After Subnetting

Before Subnetting

(4)

How to assign subnet

z

Each class can have different size of subnet field

Network

Subnet

Host

choose

appropriate size

Class A :

2 to 22 bits

Class B :

2 to 14 bits

Class C :

2 to 6 bits

Define physical subnetwork Define individual hosts

8/37

Subnet Example

z

Class B address such as 172.16.0.0 might

use its third byte to identify subnet

172. 16.

1.

0 172. 16.

2.

0 172. 16.

3.

0 172. 16.

254.

0 #1

172.16.1.1-172.16.1.254

#2

#3

#254

172.16.2.1-172.16.2.254 172.16.3.1-172.16.3.254 172.16.254.1-172.16.254.254

(5)

Example : A class B network with 24 bits mask

Subnet mask

z

subnet mask

is a 32 bit number, use to identify a subnet

255.

0.

255.

255. subnet mask= 255.255.255.0

1111 1111

0000 0000

1111 1111

Network ID

Subnet ID

Host ID

Set the bit

covering the

network and

subnet ID to 1

1

zero bit are used to mask out the host number

resulting the network address

2 10/37

1 1 0 0

1 0 1 0

0 0 0 0

1 0 0 0

0 0 0 0

0 1 0 0

0 0 0 0

Masking

1 1 0 0

1 0 1 0

0 0 0 1

0 0 0 0

0 1 0 0

0 0 0 0

0 0 1 0

1 1 1 1

0 0 0 0

& & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & &

&

172.16.4.2

255.255.255.0

172.16.4.0 A “bitwise-and” between IP address and subnet mask

yields a network address.

Note that zeros bit are used to mask out the host number

(6)

Subnet mask in Prefix format

z

The number of routing bits (network and subnet bits) in each

subnet mask can also be indicated by the "/n " format.

0000 0000 0000 0000 0000 0000 1111 1111 1100 0000 0000 0000 1111 1111 1111 1111

255.255.192.0

1111 1111 0000 0000 1111 1111 1111 1111

255.255.255.0

255.0.0.0

1111 1111 1111 0000 1111 1111 1111 1111

255.255.255.240 /8

/18

/24

/28

172.16.0.0/24

172.16.0.0 255.255.255.0 =

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Subnet routing

if dest_ip_addr & subnet_mask = = my_ip_addr & subnet_mask

send pkt on local network

%dest ip addr is on the same subnet

else

send pkt to router

%dest ip addr is on diff subnet

z

Traffic is routed to a host by looking “bit-wise and”

(7)

Routing

z

Hosts and routers perform logical AND to send packets

172.16.1.2 172.16.3.3 172.16.3.2 172.16.1.3 172.16.2.2 172.16.2.3 172.16.4.2 172.16.4.3 172.16.1.0/24 172.16.3.0/24 172.16.2.0/24 172.16.4.0/24 To 172.16.4.2

z 172.16.1.3 has a packet for 172.16.4.2 and determine that it is on other subnetwork

z The packet is sent to the router

z The router performs a subnet masking and sends the packet to the destination network

1 2

3

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Subnet interpretation

IP Address subnet mask Interpretation

15.20.15.2 255.255.0.0 host 15.2 on subnet 15.20.0.0 130.122.34.3 255.255.255.192 host 3 on subnet 130.122.34.0 130.122.34.132 255.255.255.192 host 4th on subnet 130.122.34.128 158.108.2.71 255.255.255.0 host 71 on subnet 158.108.2.0 200.190.155.66 255.255.255.192 host 2nd on subnet 200.190.155.64

(8)

Default Subnet mask

z

A default subnet mask : a subnet mask with no

subnetting

0000 0000 0000 0000 0000 0000 1111 1111

Class A

_255.0.0.0

0000 0000 0000 0000 1111 1111 1111 1111

Class B

255.255.0.0

1111 1111 0000 0000 1111 1111 1111 1111

Class C

_{255.255.255.0}

16/37

Range of bit

z

A default subnet mask : a subnet mask with no

subnetting

0

16

172 IP

0

255

255 Default subnet

255

0

255

255 New subnet

Define a subnet mask by extending the

network portion to the right, 8 bits in

this example

(9)

Computing subnet mask

Decimal equivalents of bit patterns

1 0 0 0 0 0 0 0

1 1

0 0 0 0 0 0

1 1 1

0 0 0 0 0

1 1 1 1

0 0 0 0

1 1 1 1 1

0 0 0

1 1 1 1 1 1

0 0

1 1 1 1 1 1 1

0 1 1 1 1 1 1 1 1

128

192

224

240

248

252

254

255 Binary mask

Octet value

128 64 32 16 8 4 2 1

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Compute Net and host

z

How many subnet and host are there with

172.16.0.0/24

1111 1111 0000 0000 1111 1111 1111 1111

255.

0.

255.

255. Network ID

Subnet ID

Host ID

8 bit subnet ID = 2

8

_{=256 => 254 subnets}

(10)

Network and Host relationship

z

Sample class C

71%

180

30

6

3 77%

196

14

4 49%

124

62

2

2 49%

124

2

62

6 71%

180

6

30

5 Percents

used

Total

number of

hosts

Number of

hosts

per subnet

Number of

subnets

created

Number of

subnet

bits

20/37

Subnetting Special Addresses

Subnetwork Address

All 0s

any

Purpose

HostID

NetID

Subnet-directed Broadcast

All 1s

any

Reserved addresses that are not allowed to be assigned to any node

Example: 172.16.2.0/24

Subnetwork 172.16.2.0

Example: 172.16.2.255/24

(11)

Subnet Net Block Diagram

z

Block diagram

subnetting class C

No subnetting 2 bits

3 bits 4 bits

Network Address

Broadcast Address

22/37

Contiguous and Noncontiguous mask

Noncontiguous leads to complex subnetting and routing

It is strongly recommend to use contiguous subnet mask

1111 1111

0000 0000

1111 1111

Contiguous subnet mask

000 1 1111

0000 0000

1111 1111

Noncontiguous subnet mask

no intermedite 0 gaps

in the subnet mask

intermedite 0 gaps

in the subnet mask

(12)

Subnet Class A Example

subnet mask

Interpretation

255.

0.0.0 1 network with 1677214 hosts (default subnet)

255.

255.0.0 254 subnets each with 65534 hosts

255 .

255.128.0 510 subnets

each

with 32768 hosts

255.

255.192.0 1022 subnets each with 16382 hosts

255 .

255.255.0 65534 subnets

each

with 254 hosts

24/37

#254

#2

#1

Example : Class A Subnet Address Table

10.0.0.0

10.0.0.1

10.0.255.254

10.0.255.255

10.1.0.0

10.1.0.1

10.1.255.254

10.1.255.255

10.254.0.0

10.255.0.0

10.254.0.1

10.255.0.1

10.254.255.254

10.254.255.255

10.255.255.254

10.255.255.255

10.2.0.0

10.2.0.1

10.2.255.254

10.2.255.255 IP Address : 10.0.0.0/16

Net

w

ork A

d

dress

SubnetID all 0s

SubnetID all 1s

Broadcast

A

d

dress

(13)

Class A Subnet with router

10.1.0.0

10.2.0.0

10.3.0.0

10.255.0.0 10.1.0.1 to

10.1.255.254 10.2.0.1 to

10.2.255.254 10.3.0.1 to

10.3.255.254 10.255.0.1 to

10.255.255.254 10.0.0.0/16

254 subnets each

with 65534 hosts

#1

#2

#3

#254

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Subnet Class B Example

subnet mask

Interpretation

255.255.

0.0 1 network with 65534 hosts (default subnet)

255.255.

192.0 2 subnets each with 16382 hosts

255.255.

252.0 62 subnets each with 1022 hosts

255.255.

255.0 254 subnets each with 254 hosts

255.255.

255.252 16382 subnets each with 2 hosts

(14)

#254

#2

#1

Example : Class B Subnet Address Table

172.16.0.0

172.16.0.1

172.16.0.254

172.16.0.255

172.16.1.0

172.16.1.1

172.16.1.254

172.16.1.255

172.16.254.0

176.16.255.0

172.16.254.1

176.16.255.1

176.16.254.254

176.16.254.255

176.16.255.254

176.16.255.255

172.16.2.0

172.16.2.1

172.16.2.254

172.16.2.255 IP Address : 176.16.0.0 /24

SubnetID all 0s

SubnetID all 1s

Net

w

ork A

d

dress

Broadcast

A

d

dress

28/37

Class B Subnet with router

172.16.1.0

172.16.2.0

172.16.3.0

172.16.254.0 172.16.1.1 to

172.16.1.254 172.16.2.1 to

172.16.2.254 172.16.3.1 to

172.16.3.254 172.16.254.1 to

172.16.254.254 172.16.1.0/24

254 subnets each

with 65534 hosts

#1

#2

#254

#3

(15)

Subnet Class C Example

subnet mask

Interpretation

255.255.255 .0

1 network with 254 hosts (default subnet)

255.255.255 .192

2 subnets each with 62 host

255.255.255.

224 6 subnets each with 30 hosts

255.255.255.

240 14 subnets each with 14 hosts

255.255.255.

252 62 subnets each with 2 hosts

30/37

#6

#2

#1

Example : Class C Subnet Address Table

192.68.0.0

192.68.0.1

192.68.0.30

192.68.0.31

192.68.0.32

192.68.0.33

192.68.0.62

192.68.0.63

192.68.0.192

192.68.0.224

192.68.0.193

192.68.0.225

192.68.0.222

192.68.0.223

192.68.0.254

192.68.0.255

192.68.0.64

192.68.0.65

192.68.0.94

192.68.0.95 IP Address : 192.68.0.0 /27

Net

w

ork A

d

dress

SubnetID all 0s

Broadcast

A

d

dress

SubnetID all 1s

(16)

Class C Subnet with router

192.68.0.32

192.68.0.64

192.68.0.96

192.68.0.192 192.68.0.33 to

192.68.0.62 192.68.0.65 to

192.68.0.94 192.68.0.97 to

192.68.0.126 192.68.0.193 to

192.68.0.222 192.68.0.0/27

6 subnets each

with 30 hosts

#1

#2

#6

#3

32/37

Subnet Exercise (1)

z

Given IP address 161.200, find out the following to yield not

more than 256 hosts per subnet

z

net mask= ??

z

start net id =??

z

end net id=??

z

#of subnet =??

(17)

Subnet Exercise (2)

z

Given IP address 192.150.251, find out the following to yield

not more than 32 hosts per subnet

z

net mask= ??

z

start net id =??

z

end net id=??

z

#of subnet =??

34/37

Type of Subnetting

z

all subnets in the subnetted network use

the same subnet mask

z

pros: simply to implement, easy to

maintain

z

cons: wasted address space (consider a

network of 4 hosts with 255.255.255.0

wastes 250 IP)

z

the subnets may use different subnet

masks

z

pros: utilize address spaces

z

cons: required well-management

Static Subnetting

(18)

Problem of Static subnetting

z

Inefficient allocation of

the address space

192.68.0.32/27 used 20 hosts, waste 10 hosts 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts 192.68.0.128/27 used 25 hosts, waste 5 hosts 192.68.0.192/27 used 10 hosts, waste 20 hosts 192.68.0.224/27 used 10 hosts, waste 20 hosts 36/37

Variable-Length Subnetting

192.68.0.32/27

used 20 hosts, waste 10 hosts

192.68.0.64/27

192.68.0.96/27

192.68.0.128/27

192.68.0.192/28

192.68.0.224/28

192.68.0.208/28

unused subnet Available 14 hosts

192.68.0.240/28

unused subnet Available 14 hosts z

General Idea of VLSM

z

A small subnet with only a

few hosts needs a subnet

mask that accommodate

only few hosts

z

A subnet with many hosts

need a subnet mask to

accommodate

the large number of hosts

z

General Idea of VLSM

z

A small subnet with only a

few hosts needs a subnet

mask that accommodate

only few hosts

z

A subnet with many hosts

need a subnet mask to

accommodate

(19)

VLSM - An Example

z

three different VLSM of 172.16.0.0

255.255.255.0 CPC

CPE

255.255.255.192

255.255.255.252

255.255.255.0 RDI

point-to-point link