Subnetting
Rick Graziani
Cabrillo College
Note: These example use classless addressing.
Instead of a default classful mask, a network mask is
given.
What is subnetting?
Network Network
172
16
Network Network
Host
Host
0
0
Subnet
Host
• Subnetting is the process of borrowing bits from the HOST bits, in order
•
•
•
•
to divide the larger network into small subnets.
Subnetting does NOT give you more hosts, but actually costs you hosts.
You lose two host IP Addresses for each subnet, and perhaps one for
the subnet IP address and one for the subnet broadcast IP address.
You lose the last subnet and all of it’s hosts’ IP addresses as the
broadcast for that subnet is the same as the broadcast for the network.
In older networks, you would have lost the first subnet, as the subnet IP
address is the same as the network IP address. (This subnet can be
used in most networks.)
Rick Graziani graziani@cabrillo.edu
2
Analogy
Dividing the barrel of apples into small
barrels or baskets does not give us
any more apples…
100 Apples
Rick Graziani graziani@cabrillo.edu
3
Analogy
100 Apples
(10 * 10)
•
10 barrels x 10 apples = 100 apples
10
10
10
10
10
10
10
10
10
It is the same as taking a barrel of 100
apples and dividing it into 10 barrels of
10 apples each.
Rick Graziani graziani@cabrillo.edu
10
4
Analogy
98 Apples
(100 – 2)
Rick Graziani graziani@cabrillo.edu
100 – 2 apples = 98 Usable Apples
Before subnetting:
• In any network (or subnet) we can
not use all the IP addresses for host
addresses.
• We lose two addresses for every
network or subnet.
1. Network Address - One address is
reserved to that of the network.
2. Broadcast Address – One address
is reserved to address all hosts in
that network or subnet.
5
10 barrels x 8 apples = 80 apples
8
8
(less 2)
80 Apples
10 * (10 - 2)
8
(less 2)
8
(less 2)
8
(less 2)
(less 2)
8
(less 2)
In subnetting we will see that we continue to
lose two apples per subnet, one for the
address and one for the broadcast.
Rick Graziani graziani@cabrillo.edu
(less 2)
8
8
(less 2)
•
8
(less 2)
8
6
10 barrels x 8 apples = 80 apples
--8
X
8
(less 2)
64 Apples
8 * (10 - 2)
8
(less 2)
8
(less 2)
8
(less 2)
8
(less 2)
8
(less 2)
•
8
(less 2)
8
(less 2)
We “might” also lose the last basket of apples,
subnet, as it contains the broadcast address
for the entire network.
• In older networks, we “might” also lost the first
basket, subnet, as it contained the address of
the entire network, but this is usually no longer
the case.
Rick Graziani graziani@cabrillo.edu
(less 2)
X
--8
7
Subnet Example
Network address 172.16.0.0 with /16 network mask
Network Network
172
Rick Graziani graziani@cabrillo.edu
16
Host
Host
0
0
8
Subnet Example
Network address 172.16.0.0 with /16 network mask
Network Network
Host
Host
172
16
0
0
Using Subnets: subnet mask 255.255.255.0 or /24
Network Network
Subnet
Host
Network Mask:
255.255.0.0 or /16
11111111
11111111
00000000
00000000
Subnet Mask:
255.255.255.0 or /24
11111111
11111111
11111111
00000000
•
•
Applying a mask which is larger than the default subnet
mask, will divide your network into subnets.
Subnet mask used here is 255.255.255.0 or /24
Rick Graziani graziani@cabrillo.edu
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Subnet Example
Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask 255.255.255.0 or /24
Network Network
Subnet
Host
172
172
16
16
0
1
Host
Host
172
172
172
172
16
16
16
16
2
3
Etc.
254
Host
Host
Host
Host
172
Rick Graziani graziani@cabrillo.edu
16
255
Host
Subnets
255
Subnets
28 - 1
Cannot use last
subnet as it
contains broadcast
address
10
Subnet Example
Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask 255.255.255.0 or /24
Network Network
Subnet
Host
172
172
16
16
0
1
0
0
172
172
172
172
16
16
16
16
2
3
Etc.
254
0
0
0
0
172
Rick Graziani graziani@cabrillo.edu
16
255
0
Subnets
Addresses
255
Subnets
28 - 1
Cannot use last
subnet as it
contains broadcast
address
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Subnet Example
Class B address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask 255.255.255.0 or /24
Network Network
Subnet
Hosts
Hosts
Addresses
172
172
16
16
0
1
1
1
254
254
172
172
172
172
16
16
16
16
2
3
Etc.
254
1
1
1
1
254
254
254
172
16
Rick Graziani graziani@cabrillo.edu
255
Host
254
Each subnet has
254 hosts, 28 – 2
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Subnet Example
Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask 255.255.255.0 or /24
Network Network
Subnet
Host
172
172
16
16
0
1
255
255
172
172
172
172
16
16
16
16
2
3
Etc.
254
255
255
255
255
172
Rick Graziani graziani@cabrillo.edu
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255
255
Broadcast
Addresses
255
Subnets
28 - 1
Cannot use last
subnet as it
contains broadcast
address
13
Subnet Example
Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask 255.255.255.0 or /24
172.16.0.0/24
172.16.5.0/24
Rick Graziani graziani@cabrillo.edu
172.16.10.0/24
172.16.25.0/24
14
Important things to remember about Subnetting
• You can only subnet the host portion, you do not have control of the
•
•
network portion.
Subnetting does not give you more hosts, it only allows you to divide
your larger network into smaller networks.
When subnetting, you will actually lose hosts:
– For each subnet you lose the address of that subnet
– For each subnet you lose the broadcast address of that subnet
– You “may” lose the first and last last subnets (coming)
• Analogy: Large barrel of 100 apples.
• Why subnet?
– Divide larger network into smaller network.
– Limit layer 2 and layer 3 broadcasts to their subnet.
– Better management of traffic.
Rick Graziani graziani@cabrillo.edu
15
Subnetting – Example #1 (on the board)
•
•
•
Host IP Address: 172.16.18.33
Network Mask: 255.255.0.0
Subnet Mask: 255.255.255.0
Given the following Host IP Address, Network Mask and Subnet mask find the
following information:
• Major Network Information
– Major Network Address
– Major Network Broadcast Address
– Range of Hosts if not subnetted
• Subnet Information
– Subnet Address
– Range of Host Addresses (first host and last host)
– Broadcast Address
• Other Subnet Information
– Total number of subnets
– Number of hosts per subnet
Rick Graziani graziani@cabrillo.edu
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Subnetting – Example #2
•
•
•
Host IP Address: 138.101.114.250
Network Mask: 255.255.0.0
Subnet Mask: 255.255.255.192
Given the following Host IP Address, Network Mask and Subnet mask find the
following information:
• Major Network Information
– Major Network Address
– Major Network Broadcast Address
– Range of Hosts if not subnetted
• Subnet Information
– Subnet Address
– Range of Host Addresses (first host and last host)
– Broadcast Address
• Other Subnet Information
– Total number of subnets
– Number of hosts per subnet
Rick Graziani graziani@cabrillo.edu
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Major Network Information
• Host IP Address: 138.101.114.250
• Network Mask: 255.255.0.0
• Subnet Mask: 255.255.255.192
• Major Network Address: 138.101.0.0
• Major Network Broadcast Address: 138.101.255.255
• Range of Hosts if not Subnetted: 138.101.0.1 to 138.101.255.254
Rick Graziani graziani@cabrillo.edu
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Step 1: Convert to Binary
128 64 32 16 8 4 2 1
IP Address
Mask
138.
10001010
11111111
255.
101.
01100101
11111111
255.
114.
01110010
11111111
255.
250
11111010
11000000
192
Step 1:
Translate Host IP Address and Subnet Mask into binary notation
Rick Graziani graziani@cabrillo.edu
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Step 2: Find the Subnet Address
IP Address
Mask
Network
138.
10001010
11111111
10001010
138
101.
01100101
11111111
01100101
101
114.
01110010
11111111
01110010
114
250
11111010
11000000
11000000
192
Step 2:
Determine the Network (or Subnet) where this Host address lives:
1. Draw a line under the mask
2. Perform a bit-wise AND operation on the IP Address and the Subnet
Mask
Note: 1 AND 1 results in a 1, 0 AND anything results in a 0
3. Express the result in Dotted Decimal Notation
4. The result is the Subnet Address of this Subnet or “Wire” which is
138.101.114.192
Rick Graziani graziani@cabrillo.edu
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Step 2: Find the Subnet Address
IP Address
Mask
Network
138.
10001010
11111111
10001010
138
101.
01100101
11111111
01100101
101
114.
01110010
11111111
01110010
114
250
11111010
11000000
11000000
192
Step 2:
Determine the Network (or Subnet) where this Host address lives:
Quick method:
1. Find the last (right-most) 1 bit in the subnet mask.
2. Copy all of the bits in the IP address to the Network Address
3. Add 0’s for the rest of the bits in the Network Address
Rick Graziani graziani@cabrillo.edu
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Step 3: Subnet Range / Host Range
G.D.
IP Address
Mask
Network
10001010
11111111
10001010
01100101
11111111
01100101
S.D.
01110010
11 111010
11111111
11 000000
01110010
11 000000
 subnet
  host 
counting range
counting
range
Step 3:
Determine which bits in the address contain Network (subnet)
information and which contain Host information:
• Use the Network Mask: 255.255.0.0 and divide (Great Divide) the
from the rest of the address.
• Use Subnet Mask: 255.255.255.192 and divide (Small Divide) the
subnet from the hosts between the last “1” and the first “0” in the
subnet mask.
Rick Graziani graziani@cabrillo.edu
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Step 4: First Host / Last Host
G.D.
S.D.
IP Address
Mask
Network
10001010
11111111
10001010
01100101
11111111
01100101
01110010
11 111010
11111111
11 000000
01110010
11 000000
 subnet
  host 
counting range
counting
range
First Host
10001010
138
01100101
101
01110010
114
11
000001
193
Last Host
10001010
138
01100101
101
01110010
114
11
111110
254
Broadcast
10001010
138
01100101
101
01110010
114
11
111111
255
Host Portion
• Subnet Address: all 0’s
• First Host: all 0’s and a 1
• Last Host: all 1’s and a 0
• Broadcast: all 1’s
Rick Graziani graziani@cabrillo.edu
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Step 5: Total Number of Subnets
G.D.
IP Address
Mask
Network
10001010
11111111
10001010
• TotalFirst
number
of subnets
10001010
Host
138
S.D.
01100101
11111111
01100101
01110010
11 111010
11111111
11 000000
01110010
11 000000
 subnet
  host 
counting range
counting
range
01100101
101
01110010
114
11
000001
193
– Number of subnet bits 10
10001010
01100101
01110010
11 111110
Last Host
10
– 2 = 1,024
138
101
114
254
– 1,024
total subnets
10001010
01100101
01110010
11 111111
Broadcast
101
114
255
• Subtract one138“if” all-zeros
subnet cannot
be used
• Subtract one “if” all-ones subnet cannot be used
Rick Graziani graziani@cabrillo.edu
24
Step 6: Total Number of Hosts per Subnet
G.D.
IP Address
Mask
Network
10001010
11111111
10001010
01100101
11111111
01100101
• TotalFirst
number
of hosts
10001010per subnet
01100101
Host
138
101
S.D.
01110010
11 111010
11111111
11 000000
01110010
11 000000
 subnet
  host 
counting range
counting
range
01110010
114
– Number of host bits 6
10001010
01100101
01110010
Last Host
6
– 2 = 64
138
101
114
– 64Broadcast
host per subnets
10001010
01100101
01110010
101
114
• Subtract one138for the subnet
address
• Subtract one for the broadcast address
– 62 hosts per subnet
Rick Graziani graziani@cabrillo.edu
11
000001
193
11
111110
254
11
111111
255
25
Your Turn!
Problem 1
• Host IP Address: 10.10.10.193
• Network Mask: 255.255.0.0
• Subnet Mask: 255.255.255.0
Problem 2
• Host IP Address: 10.10.10.193
• Network Mask: 255.255.255.0
• Subnet Mask: 255.255.255.240
Problem 3
• Host IP Address: 10.10.10.193
• Network Mask: 255.255.255.0
• Subnet Mask: 255.255.255.252
Rick Graziani graziani@cabrillo.edu
26
Subnetting
Rick Graziani
Cabrillo College