1. Subnetting Rick Graziani Cabrillo College
Note: These example use classless addressing. Instead of a default classful mask, a network mask is given.

2. What is subnetting? Network Network Host Host 172 16 Network Subnet
Network
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

3. Analogy
Dividing the barrel of apples into small barrels or baskets does not give us any more apples…
                                                               
                                                               
100 Apples
Rick Graziani

4. Analogy
10 barrels x 10 apples = 100 apples 10 10 10
                                                               
                                                               
                                                               
100 Apples (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. 10
Rick Graziani

5. Analogy 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.
Network Address - One address is reserved to that of the network.
Broadcast Address – One address is reserved to address all hosts in that network or subnet.
                                                               
                                                               
98 Apples (100 – 2)
Rick Graziani

6. 10 barrels x 8 apples = 80 apples
                                                               
                                                               
                                                               
(less 2)
(less 2)
(less 2)
80 Apples 10 * (10 - 2) 8 8 8
(less 2)
(less 2)
(less 2) 8 8 8
(less 2)
(less 2)
(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. 8
Rick Graziani

7. X X 10 barrels x 8 apples = 80 apples --- 8 8 8 64 Apples 8 * (10 - 2)
                                                               
                                                               
                                                               
(less 2)
(less 2)
(less 2)
64 Apples 8 * (10 - 2) 8 8 8
(less 2)
(less 2)
(less 2) 8 8 8
(less 2)
(less 2)
(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. X
--- 8
Rick Graziani

8. Subnet Example Network address 172.16.0.0 with /16 network mask
Host
Host
172 16
Rick Graziani

9. Subnet Example Network address 172.16.0.0 with /16 network mask
Host
Host
172 16
Using Subnets: subnet mask or /24
Network
Subnet
Host
Network Mask: or /16
Subnet Mask: or /24
Applying a mask which is larger than the default subnet mask, will divide your network into subnets.
Subnet mask used here is or /24
Rick Graziani

10. Subnet Example Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask or /24
Network
Subnet
Host
Subnets
172 16
Host
172 16 1
Host
172 16 2
Host
255 Subnets
28 - 1
172 16 3
Host
172 16
Etc.
Host
172 16
254
Host
Cannot use last subnet as it contains broadcast address
172 16
255
Host
Rick Graziani

11. Subnet Example Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask or /24
Network
Subnet
Host
Subnets Addresses
172 16
172 16 1
172 16 2
255 Subnets
28 - 1
172 16 3
172 16
Etc.
172 16
254
Cannot use last subnet as it contains broadcast address
172 16
255
Rick Graziani

12. Subnet Example Class B address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask or /24
Hosts Addresses
Network
Subnet
Hosts
172 16 1
254
172 16 1
254
172 16 2 1
254
172 16 3 1
254
172 16
Etc. 1
254
172 16
254 1
254
Each subnet has 254 hosts, 28 – 2
172 16
255
Host
Rick Graziani

13. Subnet Example Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask or /24
Network
Subnet
Host
Broadcast Addresses
172 16
255
172 16 1
255
172 16 2
255
255 Subnets
28 - 1
172 16 3
255
172 16
Etc.
255
172 16
254
255
Cannot use last subnet as it contains broadcast address
172 16
255
Rick Graziani

14. Subnet Example Network address 172.16.0.0 with /16 network mask
Using Subnets: subnet mask or /24
/24
/24
/24
/24
Rick Graziani

15. 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

16. Subnetting – Example #1 (on the board)
Host IP Address:
Network Mask:
Subnet Mask:
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

17. Subnetting – Example #2 Host IP Address: 138.101.114.250
Network Mask:
Subnet Mask:
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

18. Major Network Information
Host IP Address:
Network Mask:
Subnet Mask:
Major Network Address:
Major Network Broadcast Address:
Range of Hosts if not Subnetted: to
Rick Graziani

19. Step 1: Convert to Binary
Step 1:
Translate Host IP Address and Subnet Mask into binary notation
Rick Graziani

20. Step 2: Find the Subnet Address
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
Rick Graziani

21. Step 2: Find the Subnet Address
Determine the Network (or Subnet) where this Host address lives:
Quick method:
Find the last (right-most) 1 bit in the subnet mask.
Copy all of the bits in the IP address to the Network Address
Add 0’s for the rest of the bits in the Network Address
Rick Graziani

22. Step 3: Subnet Range / Host Range
Determine which bits in the address contain Network (subnet) information and which contain Host information:
Use the Network Mask: and divide (Great Divide) the from the rest of the address.
Use Subnet Mask: and divide (Small Divide) the subnet from the hosts between the last “1” and the first “0” in the subnet mask.
Rick Graziani

23. Step 4: First Host / Last Host
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

24. Step 5: Total Number of Subnets
Number of subnet bits 10
210 = 1,024
1,024 total subnets
Subtract one “if” all-zeros subnet cannot be used
Subtract one “if” all-ones subnet cannot be used
Rick Graziani

25. Step 6: Total Number of Hosts per Subnet
Number of host bits 6
26 = 64
64 host per subnets
Subtract one for the subnet address
Subtract one for the broadcast address
62 hosts per subnet
Rick Graziani

26. Your Turn! Problem 1 Host IP Address: 10.10.10.193
Network Mask:
Subnet Mask:
Problem 2
Network Mask:
Subnet Mask:
Problem 3
Subnet Mask:
Rick Graziani

27. Rick Graziani Cabrillo College
Subnetting
Rick Graziani
Cabrillo College