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Subnetting
Surasak Sanguanpong
nguan@ku.ac.th
http://www.cpe.ku.ac.th/~nguan
Last updated: 27 June 2002
Applied Network Research Group Department of Computer Engineering, Kasetsart University
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Topics
The Basics of Subnetting
Subnet Mask
Computing subnets and hosts
Subnet Routing
Creating a Subnet
Example of Subnetting
Applied Network Research Group Department of Computer Engineering, Kasetsart University

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Addressing without Subnets
172.16.1.2 172.16.1.3 172.16.2.1 172.16.254.254
172.16.0.0
A class B “Flat Network”, more than
65000 hosts
How to manage?
Performance?
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Addressing with Subnets
172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3
172.16.1.0 172.16.2.0
172.16.3.2 172.16.3.3 172.16.4.2 172.16.4.3
172.16.3.0 172.16.4.0
A class B “subdivided network”, smaller groups
with routers
Applied Network Research Group Department of Computer Engineering, Kasetsart University

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Subnetwork benefits
Smaller networks
are easier to manage Increase the network
Overall traffic is
and troubleshoot manager's control over
reduced, performance
may improve the address space
Subnetwork
Subdivide on IP network number
is an important initial task of
network managers
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Subnet Address
Before Subnetting
Network ID Host ID
After Subnetting
Network ID Subnet ID Host ID
A subnet address is created by borrowing bit
from the Host ID and designated it as a Subnet
ID field
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How to assign subnet
Each class can have different size of subnet field
Define physical subnetwork Define individual hosts
Network Subnet Host
choose
appropriate size
Class A : Class C :
2 to 22 bits Class B : 2 to 6 bits
2 to 14 bits
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Subnet Example
Class B address such as 172.16.0.0 might
use its third byte to identify subnet
Subnet Network Address Address Range
#1 172. 16. 1. 0 172.16.1.1-172.16.1.254
#2 172. 16. 2. 0 172.16.2.1-172.16.2.254
#3 172. 16. 3. 0 172.16.3.1-172.16.3.254
#254 172. 16. 254. 0 172.16.254.1-172.16.254.254
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Subnet mask
subnet mask is a 32 bit number, use to identify a subnet
Example : A class B network with 24 bits mask
Network ID Subnet ID Host ID
1
Set the bit 1111 1111 1111 1111 1111 1111 0000 0000
covering the
network and
subnet ID to 1
255. 255. 255. 0.
subnet mask= 255.255.255.0
2
zero bit are used to mask out the host number
resulting the network address
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Masking
172.16.4.2 & 255.255.255.0
1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0
& & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & &
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0
1 0 1 0 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 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.
255.0.0.0
1111 1111 0000 0000 0000 0000 0000 0000 /8
255.255.192.0
1111 1111 1111 1111 1100 0000 0000 0000 /18
255.255.255.0
1111 1111 1111 1111 1111 1111 0000 0000 /24
255.255.255.240
1111 1111 1111 1111 1111 1111 1111 0000 /28
172.16.0.0 255.255.255.0 = 172.16.0.0/24
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Subnet routing
Traffic is routed to a host by looking “bit-wise and”
results
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
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Routing
Hosts and routers perform logical AND to send packets
172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3
To 172.16.4.2
1
172.16.1.0/24 172.16.2.0/24
2
172.16.4.2 172.16.4.3
172.16.3.2 172.16.3.3
3
172.16.3.0/24 172.16.4.0/24
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
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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
Class A 255.0.0.0
1111 1111 0000 0000 0000 0000 0000 0000
Class B 255.255.0.0
1111 1111 1111 1111 0000 0000 0000 0000
Class C 255.255.255.0
1111 1111 1111 1111 1111 1111 0000 0000
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Range of bit
A default subnet mask : a subnet mask with no
subnetting
IP
172 16 0 0
Default subnet
255 255 0 0
New subnet
255 255 255 0
Define a subnet mask by extending the
network portion to the right, 8 bits in
this example
Applied Network Research Group Department of Computer Engineering, Kasetsart University

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Computing subnet mask
Decimal equivalents of bit patterns
Binary mask Octet value
128 64 32 16 8 4 2 1
1 0 0 0 0 0 0 0 128
1 1 0 0 0 0 0 0 192
1 1 1 0 0 0 0 0 224
1 1 1 1 0 0 0 0 240
1 1 1 1 1 0 0 0 248
1 1 1 1 1 1 0 0 252
1 1 1 1 1 1 1 0 254
1 1 1 1 1 1 1 1 255
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Compute Net and host
How many subnet and host are there with
172.16.0.0/24
255. 255. 255. 0.
1111 1111 1111 1111 1111 1111 0000 0000
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
Number of Number of Number of Total Percents
subnet subnets hosts number of used
bits created per subnet hosts
2 2 62 124 49%
3 6 30 180 71%
4 14 14 196 77%
5 30 6 180 71%
6 62 2 124 49%
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Subnetting Special Addresses
Reserved addresses that are not allowed to be assigned to any node
NetID HostID Purpose
any All 0s Subnetwork Address
Example: 172.16.2.0/24
Subnetwork 172.16.2.0
any All 1s Subnet-directed Broadcast
Example: 172.16.2.255/24
Directed broadcast of the subnetwork 172.16.2.0
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Subnet Net Block Diagram
No subnetting 2 bits
Block diagram
subnetting class C
3 bits 4 bits
Network Address Broadcast Address
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Contiguous and Noncontiguous mask
no intermedite 0 gaps
in the subnet mask
Contiguous subnet mask
1111 1111 1111 1111 1111 1111 0000 0000
intermedite 0 gaps
Noncontiguous subnet mask in the subnet mask
1111 1111 1111 1111 0001 1111 0000 0000
Noncontiguous leads to complex subnetting and routing
It is strongly recommend to use contiguous subnet mask
Applied Network Research Group Department of Computer Engineering, Kasetsart University

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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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Example : Class A Subnet Address Table
IP Address : 10.0.0.0/16
SubnetID all 0s
10.0.0.0 10.0.0.1 10.0.255.254 10.0.255.255
#1 10.1.0.0 10.1.0.1 10.1.255.254 10.1.255.255
Broadcast Address
Network Address
#2 10.2.0.0 10.2.0.1 10.2.255.254 10.2.255.255
#254 10.254.0.0 10.254.0.1 10.254.255.254 10.254.255.255
10.255.0.0 10.255.0.1 10.255.255.254 10.255.255.255
SubnetID all 1s
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Class A Subnet with router
10.1.0.0 10.1.0.1 to 10.0.0.0/16
#1 10.1.255.254 254 subnets each
with 65534 hosts
10.2.0.0 10.2.0.1 to
10.2.255.254
#2
#3 10.3.0.0 10.3.0.1 to
10.3.255.254
#254 10.255.0.0 10.255.0.1 to
10.255.255.254
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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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Example : Class B Subnet Address Table
IP Address : 176.16.0.0 /24
SubnetID all 0s
172.16.0.0 172.16.0.1 172.16.0.254 172.16.0.255
#1 172.16.1.0 172.16.1.1 172.16.1.254 172.16.1.255
Broadcast Address
Network Address
#2 172.16.2.0 172.16.2.1 172.16.2.254 172.16.2.255
#254 172.16.254.0 172.16.254.1 176.16.254.254 176.16.254.255
176.16.255.0 176.16.255.1 176.16.255.254 176.16.255.255
SubnetID all 1s
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Class B Subnet with router
#1 172.16.1.0 172.16.1.1 to 172.16.1.0/24
172.16.1.254 254 subnets each
172.16.2.0 172.16.2.1 to with 65534 hosts
172.16.2.254
#2
#3 172.16.3.0 172.16.3.1 to
172.16.3.254
#254 172.16.254.0 172.16.254.1 to
172.16.254.254
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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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Example : Class C Subnet Address Table
IP Address : 192.68.0.0 /27
SubnetID all 0s
192.68.0.0 192.68.0.1 192.68.0.30 192.68.0.31
#1 192.68.0.32 192.68.0.33 192.68.0.62 192.68.0.63
Broadcast Address
Network Address
#2 192.68.0.64 192.68.0.65 192.68.0.94 192.68.0.95
#6 192.68.0.192 192.68.0.193 192.68.0.222 192.68.0.223
192.68.0.224 192.68.0.225 192.68.0.254 192.68.0.255
SubnetID all 1s
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Class C Subnet with router
192.68.0.32 192.68.0.33 to 192.68.0.0/27
#1 192.68.0.62 6 subnets each
192.68.0.64 192.68.0.65 to with 30 hosts
192.68.0.94
#2
#3 192.68.0.96 192.68.0.97 to
192.68.0.126
#6 192.68.0.192 192.68.0.193 to
192.68.0.222
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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 =??
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Type of Subnetting
Static 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)
Variable Lengh Subnetting
the subnets may use different subnet
masks
pros: utilize address spaces
cons: required well-management
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Problem of Static 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
Inefficient allocation of
192.68.0.128/27
used 25 hosts, the address space
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
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Variable-Length Subnetting
192.68.0.32/27 General Idea of VLSM
used 20 hosts, General Idea of VLSM
waste 10 hosts A small subnet with only a
A small subnet with only a
few hosts needs a subnet
few hosts needs a subnet
192.68.0.64/27
used 20 hosts, mask that accommodate
mask that accommodate
waste 10 hosts only few hosts
only few hosts
192.68.0.96/27 A subnet with many hosts
A subnet with many hosts
used 25 hosts, need a subnet mask to
need a subnet mask to
waste 5 hosts accommodate
accommodate
the large number of hosts
the large number of hosts
192.68.0.128/27
used 25 hosts,
waste 5 hosts
192.68.0.192/28 192.68.0.208/28
used 10 hosts, unused subnet
waste 4 hosts Available 14 hosts
192.68.0.224/28 192.68.0.240/28
used 10 hosts, unused subnet
waste 4 hosts Available 14 hosts
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VLSM - An Example
three different VLSM of 172.16.0.0
CPC RDI
255.255.255.0 255.255.255.0
255.255.255.252
255.255.255.252 255.255.255.252
point-to-point link
CPE
255.255.255.252
255.255.255.192
Applied Network Research Group Department of Computer Engineering, Kasetsart University