1. Chapter 12
Subnetting
CIS 1140

2. Subnetting
Subnetting: taking a single network address issued from a registrar or an ISP and creating two or more subnetworks
A subnet is a network created by borrowing bits from the host portion of an assigned network IP address.
Binary number system is used for IP addressing.
Even though binary number system is used for addressing, IP addresses are typically written in dotted decimal format. Ex

3. Subnetting (cont.)
1. Determine the class of the IP address to be used and all accompanying information:
0 – 127 Class A
N – H – H – H
128 – 191 Class B
N – N – H – H
192 – 223 Class C
N – N – N – H

4. Subnetting (cont.)
2. Determine how many bits to “borrow” for subnetting based on the number of networks or hosts that must be accommodated
You must leave at least 2 bits for hosts.
2n - 2 ≥ number of subnets needed (where n is the number of bits borrowed for subnetting)
2n - 2 ≥ number of hosts needed (where n is the number of bits left for addressing hosts)

5. Subnetting (cont.)
How many bits would need to be borrowed to produce 7 subnets?
2n - 2 ≥ 7
2n ≥ 9
n = 4 (remember 24=16)
How many bits would need to be borrowed to produce 25 subnets?
2n - 2 ≥ 25
2n ≥ 27
n = 5 (remember 25=32)

6. Subnetting (cont.)
How many bits would need to be borrowed to produce 220 subnets?
2n - 2 ≥ 220
2n ≥ 222
n = 8 (remember 28=256)
How many bits would need to be used to produce 12 hosts per subnet?
2n - 2 ≥ 12
2n ≥ 14
n = 4 (remember 24=16)

7. Subnetting (cont.)
How many bits would need to be used to produce 35 hosts per subnet?
2n - 2 ≥ 35
2n ≥ 37
n = 6 (remember 26=64)
How many bits would need to be used to produce 500 hosts per subnet?
2n - 2 ≥ 500
2n ≥ 502
n = 9 (remember 29=512)

8. Subnetting (cont.)
How many bits are borrowed for subnetting a class B address if 10 must be used for hosts?
8 bits from the 3rd octet + 8 bits from the 4th octet give 16 bits for hosts + subnets
16 – 10 = 6
How many bits are borrowed for subnetting a class C address if 3 must be used for hosts?
8 bits from the 4th octet give 8 bits for hosts + subnets
8 – 3 = 5

9. Subnetting (cont.)
3. Determine the actual subnet mask of the newly created networks
Start with the default subnet mask and add in the bits that were “borrowed” or set during the subnetting process
If three bits were borrowed from a class C address:
= 224 ( )
The mask would be

10. Subnetting (cont.) If four bits are borrowed from a class C address:
= 240 ( )
The mask would be
If four bits are borrowed from a class B address:
The mask would be

11. Subnetting (cont.)
4. Determine what value you are ranging on. The last bit turned on in an octet will determine the range.
Borrow 3 bits for subnetting
This would range on 32.
*Note the equation 2n - 2 ≥ subnets When n = 3 it is predicted that 6 useable subnets will be produced. When the first and last subnet are thrown out this is true. This would leave 5 bits for hosts. 2n - 2 ≥ hosts would predict 30 useable hosts which is also true.

12. Subnetting (cont.)
The first address in a network or subnet is used to identify the subnet/network and cannot be used to address hosts.
The last address in a network or subnet is used as the broadcast for the subnet/network and cannot be used to address hosts.
When a subnet is created, we typically throw out the first and last subnet to avoid confusion:
In the previous example:
Does identify the entire network or just the first subnet?
Does create a broadcast for the entire network or just the subnet ?
This throwing out of two subnets and two addresses in each subnet explains the – 2 in the equation 2n - 2 ≥ number of hosts/subnets

13. Subnetting (cont.) Borrow 4 bits for subnetting 192.168.3.0
This would range on 16.
*Note the equation 2n - 2 ≥ subnets When n = 4 it is predicted that 14 useable subnets will be produced. When the first and last subnet are thrown out this is true. This would leave 4 bits for hosts. 2n - 2 ≥ hosts would predict 14 useable hosts which is also true.

14. Variable Length Subnet Mask
Variable Length Subnet Mask (VLSM): a subnet mask that is not expressed in standard 8-bit or one-byte values. A VLSM occurs when a subnet is further divided into smaller subnets, which are not equal to the original subnet in length or in the number of hosts.
Subnetting a subnet
Avoids wasting addresses when subnets vary greatly in size
Ex. Point-to-point links really require only 4 address: 1 ID 1 Broadcast 2 one address for each side of the link
Fixed Length subnet Mask (FLSM): subnets are equal in range and have an equal number of hosts

15. Advantages of Subnetting
Creates a more secure network by placing hosts on separate networks.
Breaks up broadcast domains. Broadcast traffic from one network does not “bleed” over into other networks.
Fewer collision occur because of reduced broadcasts and segmentation reduces the size of the collision domains.

16. Disadvantages of Subnetting
Subnetting can become confusing as networks grow larger.
Subnetting itself is not an easy process if the administrator does not practice.
Not all device and protocols can use all types of subnetting.
Ex. Not all routing protocols support VLSM.