Topics The Basics of Subnetting Subnet Mask Computing subnets and - - PDF document

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Subnetting

Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan

Last updated: 27 June 2002

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Topics

The Basics of Subnetting Subnet Mask Computing subnets and hosts Subnet Routing Creating a Subnet Example of Subnetting

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Addressing without Subnets

A class B “Flat Network”, more than

65000 hosts

How to manage? Performance?

172.16.1.2 172.16.1.3 172.16.254.254 172.16.2.1

172.16.0.0

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Addressing with Subnets

172.16.1.2 172.16.3.3 172.16.3.2 172.16.1.3

• 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

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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

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Subnet Address

• 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

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How to assign subnet

• 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

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Subnet Example

Class B address such as 172.16.0.0 might

use its third byte to identify subnet

172. 16. 1. 172. 16. 2. 172. 16. 3. 172. 16. 254.

#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 mask

• 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 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

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1 1 0 0 1 0 1 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Masking

1 1 0 0 1 0 1 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 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 format

• 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

• 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

172.16.1.2 172.16.3.3 172.16.3.2 172.16.1.3 172.16.2.2 172.16.2.3 172.16.4.2 172.16.4.3

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

• 172.16.1.3 has a packet for 172.16.4.2 and determine

that it is on other subnetwork

• The packet is sent to the router
• 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

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Default Subnet mask

• 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

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Range of bit

• A default subnet mask : a subnet mask with no

subnetting

16 172

IP

255 255

Default subnet

255 255 255

New subnet

Define a subnet mask by extending the network portion to the right, 8 bits in this example

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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 Applied Network Research Group Department of Computer Engineering, Kasetsart University

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Compute Net and host

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

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Network and Host relationship

Sample class C

71% 180 30 6 3 77% 196 14 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

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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 Directed broadcast of the subnetwork 172.16.2.0

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Subnet Net Block Diagram

Block diagram

subnetting 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 0000 1111 1111 1111 1111 Contiguous subnet mask 0001 1111 0000 0000 1111 1111 1111 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 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

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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

Network Address SubnetID all 0s SubnetID all 1s Broadcast Address

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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

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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 Network Address Broadcast Address

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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

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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

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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

Network Address SubnetID all 0s Broadcast Address SubnetID all 1s

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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

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Subnet Exercise (1)

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

more 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 yield

not more than 32 hosts per subnet

net mask= ?? start net id =?? end net id=?? #of subnet =?? Applied Network Research Group Department of Computer Engineering, Kasetsart University

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Type of Subnetting

• all subnets in the subnetted network use

the same subnet mask

• pros: simply to implement, easy to

maintain

• cons: wasted address space (consider a

network of 4 hosts with 255.255.255.0 wastes 250 IP)

• the subnets may use different subnet

masks

• pros: utilize address spaces
• cons: required well-management

Static Subnetting Variable Lengh Subnetting

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Problem of Static subnetting

• 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University

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Variable-Length Subnetting

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

192.68.0.192/28

used 10 hosts, waste 4 hosts

192.68.0.224/28

used 10 hosts, waste 4 hosts

192.68.0.208/28

unused subnet Available 14 hosts

192.68.0.240/28

unused subnet Available 14 hosts

• General Idea of VLSM
• A small subnet with only a

few hosts needs a subnet mask that accommodate

• nly few hosts
• A subnet with many hosts

need a subnet mask to accommodate the large number of hosts

• General Idea of VLSM
• A small subnet with only a

few hosts needs a subnet mask that accommodate

• nly few hosts
• A subnet with many hosts

need a subnet mask to accommodate the large number of hosts

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VLSM - An Example

• three different VLSM of 172.16.0.0

255.255.255.0 CPC CPE 255.255.255.192 255.255.255.252 255.255.255.252 255.255.255.252 255.255.255.252 255.255.255.0 RDI point-to-point link