Classless Subnetting
Using the Worksheet
CIS 81 and CST 311
Rick Graziani
Fall 2006
Determining Network and Subnet Information
• Use the Classless Subnetting Worksheet (Excel Spreadsheet) to do
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the following:
Given any IP address and major network mask we can determine:
– Major Network Address
– First host address of the network
– Last host address of the network
– Broadcast address of the network
– Number of usable hosts in the network
If the network is subnetted and we know the subnet mask we can
determine:
– Subnet (network) Address
– First host address of the subnet
– Last host address of the subnet
– Broadcast address of the subnet
– Number of usable hosts in the subnet
– Number of usable subnets in this network
Rick Graziani graziani@cabrillo.edu
2
See these spreadsheets on my website
Nutshell: Classless
Subnetting in a Nutshell
(Excel spreadsheet)
Worksheet: Classless
Subnetting Worksheet (Excel
spreadsheet)
Rick Graziani graziani@cabrillo.edu
3
Part 1: Determine Major Network Information
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Convert IP Address/Network Mask to Binary
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First, let’s determine the Major Network Information.
This is the information for the entire network, whether or not there are subnets.
Using the Major Network Mask, determine the major network Address, the broadcast
address for the entire network, and the number of hosts for the entire network.
The Major Network Mask (or Base Mask, Subnet Mask) is provided by the ISP.
Convert these addresses to binary.
Host IP Address:138.101.114.250
Major Network Mask: 255.255.0.0 (/16)
Rick Graziani graziani@cabrillo.edu
5
Determine Network Address
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Determine the Network Address by using the AND operation.
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
Express the result in Dotted Decimal Notation
The result is the Major Network Address of this for this host IP Address is
138.101.0.0
Rick Graziani graziani@cabrillo.edu
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Determine Network Address
A simple way of doing the AND operation:
1. In the Network mask locate where the 1’s end and the 0’s begin and draw a
line. (I call this the “Major Network Divide” or “MD” on the worksheet.)
2. Now copy all of the bits above the 1 bits in the Network mask, to the Network
address.
3. For the rest of the bits in the Network address (the bits below the 0’s in the
Network mask) write all 0’s.
MD
Network Portion
Copy the bits from the Host IP Address
to the Major Network Address
Rick Graziani graziani@cabrillo.edu
Host Portion
Write all 0’s below the 0’s in
the Major Network Mask
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Network: Determine First Host, Last Host,
Broadcast Addresses
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Remember that the network mask separates the network portion of the address from the
host portion.
Major Network Mask: 255.255.0.0 or /16
The network address has all 0’s in the host portion of the address
The broadcast address has all 1’s in the host portion of the address
The first host is all 0’s and a 1 in the host portion of the address.
The last host is all 1’s and a 0 in the host portion of the address.
Network Portion
Rick Graziani graziani@cabrillo.edu
Host Portion
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Network: Determine First Host, Last Host,
Broadcast Addresses
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The network address has all 0’s in the host portion of the address
The broadcast address has all 1’s in the host portion of the address
The first host is all 0’s and a 1 in the host portion of the address.
The last host is all 1’s and a 0 in the host portion of the address.
Network Portion
Rick Graziani graziani@cabrillo.edu
Host Portion
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Network: Determine the number of usable hosts
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By counting the number of host bits we can determine the total number of usable hosts
for this network (before subnetting).
Host bits: 16
Total number of hosts:
216 = 65,536
65,536 – 2 = 65,534 (Can’t use the all 0’s address, network address, or the all 1’s
address, broadcast address.)
Network Portion
Host Portion = 16 bits
Rick Graziani graziani@cabrillo.edu
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Part 2: Determine Subnet Information
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Convert IP Address/Subnet Mask to Binary
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Now we will determine the Subnet Network Information. (Assuming we are subnetted.)
This is the information only for that subnet.
Using the Subnet Mask, determine the Subnet Address, the broadcast address for the
entire network, and the number of hosts for the subnet.
The Subnet Mask is determined by the network administrator, depending upon the
number of subnets and the number of hosts per subnet that are needed.
Convert these addresses to binary.
Host IP Address:138.101.114.250
Subnet Mask: 255.255.255.192 (/26)
Rick Graziani graziani@cabrillo.edu
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Determine Subnet Address
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Determine the Network Address by using the AND operation.
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
Express the result in Dotted Decimal Notation
The result is the Major Network Address of this for this host IP Address is
138.101.114.192
Rick Graziani graziani@cabrillo.edu
13
Determine Subnet Address
A simple way of doing the AND operation:
1. In the Subnet mask locate where the 1’s end and the 0’s begin and draw a line.
(I call this the “Subnet Divide” or “SD” on the worksheet.)
2. Now copy all of the bits above the 1 bits in the Subnet mask, to the Network
address.
3. For the rest of the bits in the Subnet address (the bits below the 0’s in the
Network mask) write all 0’s.
SD
Network/Subnet Portion
Copy the bits from the Host IP
Address to the Major Network
Address
Rick Graziani graziani@cabrillo.edu
Host
Portion
Write all 0’s below the 0’s in
the Major Network Mask
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Subnet: Determine First Host, Last Host,
Broadcast Addresses
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Remember that the network mask separates the network portion of the address from the
host portion.
Subnet Mask: 255.255.255.192 or /26
The network address has all 0’s in the host portion of the address
The broadcast address has all 1’s in the host portion of the address
The first host is all 0’s and a 1 in the host portion of the address.
The last host is all 1’s and a 0 in the host portion of the address.
Network Portion
Subnet Portion
Host
Portion
Rick Graziani graziani@cabrillo.edu
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Subnet: Determine First Host, Last Host,
Broadcast Addresses
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The subnet address has all 0’s in the host portion of the subnet address
The broadcast address has all 1’s in the host portion of the subnet address
The first host is all 0’s and a 1 in the host portion of the subnet address.
The last host is all 1’s and a 0 in the host portion of the subnet address.
Network Portion
Subnet Portion
Host
Portion
Rick Graziani graziani@cabrillo.edu
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Subnet: Determine the number of usable hosts
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By counting the number of host bits we can determine the total number of usable hosts
for this subnet.
Host bits: 6
Total number of hosts:
26 = 64
64 – 2 = 62 (Can’t use the all 0’s address, network address, or the all 1’s
address, broadcast address.)
SD
Network Portion
Rick Graziani graziani@cabrillo.edu
Subnet Portion
Host
Portion
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Subnet: Determine the number of usable
subnets
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By counting the number of subnet bits we can determine the total number of usable hosts for this
subnet.
Subnet bits: 10
Total number of hosts:
210 = 1,024
1,024 – (0, 1, or 2) = ?
1,024 – 1 = 1,023 usable subnets
The number of usable subnets depends upon whether or not we can use the first and/or last subnets.
In today’s networks, both the first and last subnets are generally usable.
In this example, the network administrator has determined the last subnet is not to be used.
MD
Network Portion
Rick Graziani graziani@cabrillo.edu
Subnet Portion
SD
Host
Portion
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Overall Visual
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The subnet address has all 0’s in the host portion of the subnet address
The broadcast address has all 1’s in the host portion of the subnet address
The first host is all 0’s and a 1 in the host portion of the subnet address.
The last host is all 1’s and a 0 in the host portion of the subnet address.
Rick Graziani graziani@cabrillo.edu
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Overall Visual
The following information must be provided:
• IP Address (host or network)
• Major Network Mask
If subnetted:
• Subnet Mask
• Number of usable subnets (less 0, 1, or 2)
Rick Graziani graziani@cabrillo.edu
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Notes
Quick check
• First host: 1 more than network/subnet address
• Last host: 1 less than broadcast
• Does the host IP address fall in the range of network host addresses? Of subnet host addresses?
How do hosts view the network?
• Hosts receive a host IP address and mask.
• Hosts only see themselves as part of their subnet (or network if not subnetted).
• They don’t know or care if they are in a network or subnet.
• Almost all networks are a subnet of some larger network.
NAP (Network Access Point)
Network
Service
Provider
Regional
Service
Provider
ISP
Subscribers
ISP
Subscribers
Rick Graziani graziani@cabrillo.edu
ISP
Subscribers
Network
Service
Provider
Regional
Service
Provider
Regional
Service
Provider
ISP
ISP
Subscribers
Subscribers
Regional
Service
Provider
ISP
Subscribers
ISP
Subscribers
ISP
Subscribers
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See these spreadsheets on my website
Nutshell: Classless
Subnetting in a Nutshell
(Excel spreadsheet)
Worksheet: Classless
Subnetting Worksheet (Excel
spreadsheet)
Rick Graziani graziani@cabrillo.edu
22
Tips
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Use worksheets
Don’t do short-cuts unless you understand the process we
just discussed and you know what you are doing.
Only use a subnet calculator to check your answers.
– You must know how to subnet, then you can use the
calculator.
– Interviews, exams, and certification exams do not allow
subnet calculators.
Practice, practice, practice!
Rick Graziani graziani@cabrillo.edu
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Classless Subnetting
Using the Worksheet
CIS 81 and CST 311
Rick Graziani
Fall 2006