Subnetting - Kasetsart Universitynguan/class/204421/slides/subnet.pdfApplied Network Research Group Department of Computer Engineering, Kasetsart University 1/37 Subnetting Surasak

Applied Network Research Group Department of Computer Engineering, Kasetsart University

1/37

Subnetting

Surasak [email protected]

http://www.cpe.ku.ac.th/~nguan

Last updated: 27 June 2002


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Topics

The Basics of SubnettingSubnet MaskComputing subnets and hostsSubnet RoutingCreating a SubnetExample of Subnetting


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Addressing without Subnets

A class B “Flat Network”, more than65000 hosts

How to manage?Performance?

172.16.1.2 172.16.1.3 172.16.254.254172.16.2.1

172.16.0.0


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Addressing with Subnets172.16.1.2

172.16.3.3172.16.3.2

172.16.1.3

A class B “subdivided network”, smaller groupswith 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


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Subnetwork

Subnetwork benefits

Subdivide on IP network numberis an important initial task of

network managers

Increase the network manager's control over

the address space

Smaller networksare easier to manage

and troubleshootOverall traffic is reduced, performance

may improve


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Subnet Address

A subnet address is created by borrowing bitfrom the Host ID and designated it as a SubnetID field

Network ID Subnet ID Host ID

Network ID Host ID

After Subnetting

Before Subnetting


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How to assign subnetEach 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


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Subnet Example

Class B address such as 172.16.0.0 mightuse 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

Subnet Network Address Address Range


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Example : A class B network with 24 bits mask

Subnet masksubnet mask is a 32 bit number, use to identify a subnet

255. 0.255.255.

subnet mask= 255.255.255.0

1111 1111 0000 00001111 11111111 1111


Set the bitcovering thenetwork andsubnet ID to 1

1

zero bit are used to mask out the host number resulting the network address

2


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1 1 0 01 0 1 0 1 0 0 00 0 0 0 0 1 0 00 0 0 0 0 0 0 00 0 0 0

Masking

1 1 0 01 0 1 0 0 0 0 00 0 0 1 0 1 0 00 0 0 0 0 0 1 00 0 0 0

1 1 1 11 1 1 1 0 0 0 00 0 0 01 1 1 11 1 1 1 1 1 1 11 1 1 1& & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & &

&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

resulting the network address


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Subnet mask in Prefix formatThe number of routing bits (network and subnet bits) in eachsubnet mask can also be indicated by the "/n " format.

0000 0000 0000 00000000 00001111 1111

1100 0000 0000 00001111 11111111 1111

255.255.192.0

1111 1111 0000 00001111 11111111 1111

255.255.255.0

255.0.0.0

1111 1111 1111 00001111 11111111 1111

255.255.255.240

/8

/18

/24

/28

172.16.0.0/24172.16.0.0 255.255.255.0 =


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Subnet routing

if dest_ip_addr & subnet_mask = = my_ip_addr & subnet_masksend pkt on local network %dest ip addr is on the same subnet

else send pkt to router %dest ip addr is on diff subnet

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


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Routing

Hosts and routers perform logical AND to send packets

172.16.1.2

172.16.3.3172.16.3.2

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

172.16.1.3 has a packet for 172.16.4.2 and determinethat it is on other subnetworkThe packet is sent to the routerThe router performs a subnet masking and sends thepacket 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


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Default Subnet maskA default subnet mask : a subnet mask with nosubnetting

0000 0000 0000 00000000 00001111 1111

Class A 255.0.0.0

0000 0000 0000 00001111 11111111 1111

Class B 255.255.0.0

1111 1111 0000 00001111 11111111 1111

Class C 255.255.255.0


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Range of bitA default subnet mask : a subnet mask with nosubnetting

0 016172

IP

0 0255255

Default subnet

255 0255255

New subnet

Define a subnet mask by extending thenetwork portion to the right, 8 bits inthis example


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Computing subnet mask

Decimal equivalents of bit patterns

1 0 0 0 0 0 0 01 1 0 0 0 0 0 01 1 1 0 0 0 0 01 1 1 1 0 0 0 01 1 1 1 1 0 0 01 1 1 1 1 1 0 01 1 1 1 1 1 1 01 1 1 1 1 1 1 1

128192224240248252254255

Binary mask Octet value

128 64 32 16 8 4 2 1


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Compute Net and hostHow many subnet and host are there with172.16.0.0/24

1111 1111 0000 00001111 11111111 1111

255. 0.255.255.


8 bit subnet ID = 28=256 => 254 subnets8 bit host ID = 28=256 => 254 hosts per subnet


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Network and Host relationshipSample class C

71%1803063

77%19614144

49%1246222

49%1242626

71%1806305

Percentsused

Totalnumber of

hosts

Number ofhosts

per subnet

Number ofsubnetscreated

Number ofsubnet

bits


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Subnetting Special Addresses

Subnetwork AddressAll 0sany

PurposeHostIDNetID

Subnet-directed BroadcastAll 1sany

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 Directed broadcast of the subnetwork 172.16.2.0


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Subnet Net Block Diagram

Block diagramsubnetting class C

No subnetting 2 bits

3 bits 4 bits

Network Address Broadcast Address


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Contiguous and Noncontiguous mask

Noncontiguous leads to complex subnetting and routing

It is strongly recommend to use contiguous subnet mask

1111 1111 0000 00001111 11111111 1111

Contiguous subnet mask

0001 1111 0000 00001111 11111111 1111

Noncontiguous subnet mask

no intermedite 0 gaps in the subnet mask

intermedite 0 gaps in the subnet mask


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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





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#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

Netw

ork Address

SubnetID all 0s

SubnetID all 1s

Broadcast A

ddress


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Class A Subnet with router10.1.0.0

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

10.0.0.0/16254 subnets each with 65534 hosts

#1

#2

#3

#254


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Subnet Class B Example








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#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

Netw

ork Address

Broadcast A

ddress


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Class B Subnet with router172.16.1.0

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


#1

#2

#254

#3


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Subnet Class C Example



255.255.255.192 2 subnets each with 62 host





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#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

Netw

ork Address

SubnetID all 0s

Broadcast A

ddress

SubnetID all 1s


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Class C Subnet with router192.68.0.32

192.68.0.64

192.68.0.96

192.68.0.192

192.68.0.33 to192.68.0.62

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


#1

#2

#6

#3


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Subnet Exercise (1)Given IP address 161.200, find out the following to yield notmore than 256 hosts per subnet

net mask= ??start net id =??end net id=??#of subnet =??


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Subnet Exercise (2)Given IP address 192.150.251, find out the following to yieldnot more than 32 hosts per subnet

net mask= ??start net id =??end net id=??#of subnet =??


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Type of Subnetting

all subnets in the subnetted network usethe same subnet mask

pros: simply to implement, easy tomaintaincons: wasted address space (consider anetwork of 4 hosts with 255.255.255.0wastes 250 IP)

the subnets may use different subnetmasks

pros: utilize address spacescons: required well-management

Static Subnetting

Variable Lengh Subnetting


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Problem of Static subnetting

Inefficient allocation ofthe address space

192.68.0.32/27used 20 hosts, waste 10 hosts







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Variable-Length Subnetting192.68.0.32/27used 20 hosts,waste 10 hosts

192.68.0.64/27used 20 hosts,waste 10 hosts





192.68.0.208/28unused subnetAvailable 14 hosts

192.68.0.240/28unused subnetAvailable 14 hosts

General Idea of VLSMA small subnet with only afew hosts needs a subnetmask that accommodateonly few hostsA subnet with many hostsneed a subnet mask toaccommodatethe large number of hosts

General Idea of VLSMA small subnet with only afew hosts needs a subnetmask that accommodateonly few hostsA subnet with many hostsneed a subnet mask toaccommodatethe large number of hosts


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VLSM - An Examplethree different VLSM of 172.16.0.0

255.255.255.0

CPC

CPE

255.255.255.192

255.255.255.252

255.255.255.252

255.255.255.252

255.255.255.252

255.255.255.0

RDI

point-to-point link

Documents

Subnetting - Kasetsart Universitynguan/class/204421/slides/subnet.pdfApplied Network Research Group Department of Computer Engineering, Kasetsart University 1/37 Subnetting Surasak