Subnetting Process
1. Introduction
The process of subnetting is both a mathematical process and a network design process. Mathematics drives how
subnets are calculated, identified, and assigned. The network design and requirements of the organization drive
how many subnets are needed and how many hosts an individual subnet needs to support. In addition, basic
knowledge of binary number manipulation including converting binary numbers to decimal and vice versa is
essential part of subnetting process. The following sections provide basic definitions of the subnetting
terminologies. For binary arithmetic you can use Windows calculator’s Programmer feature as shown in Fig. 1.
Fig. 1. Windows 10 calculator Programmer mode.
2. IP address Classes
IP addresses are divided into three classes: A, B, and C.
• Class A: IP addresses are those between 0.0.0.0 and 127.255.255.255.
• Class B: IP addresses are those between 128.0.0.0 and 191.255.255.255.
• Class C: IP addresses are those between 192.0.0.0 and 223.255.255.255.
The format shown above is called dotted or octet format.
To determine the class of an IP address, look at the first number:
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•
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If the first number is 1 through 127, it will be a class A address.
If the first number is 128 through 191, it is a class B address.
If the first number is 192 through 223, it is a class C address.
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3. Subnetting
IPv4 addresses is 32-bit long. That means we can have 2^32 (2 to the power of 32) (4,294,967,296) addresses,
which is not enough. To increase the number of available IP addresses we must use Subnetting. Subnetting is the
process of partitioning a single IP address into several separate networks called subnets. There are two parts in an
IP address, i.e., Network part and Host part. With subnetting, we borrow some bits from the host part and add
them to the network part. These borrowed bits are shown as Subnet Bits in Fig.2
Fig.2. Subnetting schematic
4. Subnet Masks
The technique used to create subnets is to use a Subnet Mask. Subnets masks are 32-bit addresses like IP
Addresses. The 1s represents the network parts, and 0s represents the host parts. We can show Subnet Masks with
four octets like IP addresses (255.255.255.0) or we can show it like /X, which is called prefix. For example, for the
255.255.255.0 Subnet Mask, the prefix format is /24. This means that the first 24 bit is all 1s and it is network part,
and the remaining 8 bit is all 0s and it is host part.
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5.
Subnetting Example
Table 1.
Requirements
The IP address to be subnetted
Number of required subnets
Determine number of host bits to borrow
Results
213.190.21.0/24
6
n=3
(2^n = 2^3 = 8 > 6, where n = the number of host bits to borrow
Calculating new subnet mask
See table 2
Apply the LSB (see Table 2) to the new
See Table 3
subnet mask to get the Subnet addresses.
Table 2. Calculating the new subnet mask
Action
IP address
Number of host bits available
Number of bits borrowed
Number of remaining hots bits
Original subnet mask
Original subnet mask prefix
New subnet mask
New prefix
The LSB (the least significant bit) is
the last bit borrowed
Results
213.190.21.0/24
8
3
5
255.255.255.0
/24
255.255.255.224
/27
Shown in red font
In binary
11010101.10111110.00010101.00000000
11111111.11111111.1111111.00000000
11111111.11111111.1111111.11100000
In this example, this bit has the value of 2^5 = 32
In a simple statement, explain how you are calculating each of the following. One or at most two lines for each
one. They are clearly explained in the reference provided below.
LSB (Least Significant Bit)
Subnet addresses
Broadcast addresses
Host range for each subnet
You must show how get the results. Just showing the results is not sufficient.
For this example, the above information for subnet numbers 1, 2, & 3 are provided in Table 3.
Table 3. Results
Subnet #
1
2
3
4
Subnet Addresses
213.190.21.0
213.190.21.32
213.190.21.64
213.190.21.96
Host range
213.190.21.1-30
213.190.21.33-62
213.190.21.65-94
213.190.21.97-126
Broadcast address
213.190.21.31
213.190.21.63
213.190.21.95
213.190.21.127
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Note:
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•
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Your deliverable must include the above three table for your selected problem and only for subnets 1, 2,
3, & 4. In another word, it would be about one page, like page 3 above.
The following reference provides all the necessary information you need for subnetting. Please take time
and read it very carefully before you attempting to work on the assignment.
There are numerous Internet resources on this topic that explain subnetting in various ways. Some of
these methods may be advanced and beyond the scope of this assignment. You are required to focus only
on the method presented here.
The above discussion assumes you know basics of binary numbers including converting decimal numbers
to binary and vice-versa. If you are not familiar with the process, please use the URLs embedded in the
project 1 (and possibly other resources) to learn basics about binary number manipulation.
Below, I have provided some additional binary arithmetic for this example. You do not have to submit
these details. It is only for your curiosity.
References
Subnetting an IP address
https://d12vzecr6ihe4p.cloudfront.net/media/966010/wp-subnetting-an-ip-address.pdf
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