An subnet part that is used by internal routers


An Subnet part that is used by internal routers within
the organization to deliver packets to a particular smaller
Subnet.

An IP Host part that identifies a particular individual
device.
TDC 594

Subnetting Example Based on
Subnet Requirements

BrewCo was allocated IP address block
201.16.72.0/24 by their ISP. They have 6
departments and want to use 8 subnets
internally. What Subnet Mask should they use?

Initial subnet = 201.16.72.0 / 24

Initial prefix, subnet mask = /24 or 255.255.255.0

Size of initial subnet = 2
(32-24)
= 2
8
= 256 IP addresses

Number of new subnets = 8 = 2 3

New prefix length = 24 + 3 = / 27

New subnet mask = 255.255.255.224

Size of each new subnet = 2
(32-24-3)
= 2
5
= 32

Result: Original block of 256 addresses is split
into 8 subnets of 32 IP addresses each .

Listing New Subnets

Original subnet = 201.16.72.0/24 .

Using mask 255.255.255.224, the 8 new subnets
are:

Subnet 201.16.72.0 /27

Valid host range = 201.16.72.1 – 201.16.72.30

Subnet broadcast address = 201.16.72.31

Subnet 201.16.72.32 /27

Valid host range = 201.16.72.33 – 201.16.72.62

Subnet broadcast address = 201.16.72.63

Subnet 201.16.72.64 /27

Valid host range = 201.16.72.65 – 201.16.72.94

Subnet broadcast address = 201.16.72.63

(… leaving four subnets out here …)

Subnet 201.16.72.224 /27

Valid host range = 201.16.72.225 – 201.16.72.254

Subnet broadcast address = 201.16.72.255

Determining the Subnet Mask
Subnetting Based on
Subnet or Host Requirements
There are two considerations when
planning subnets:

Number of Subnets required

Number of Host addresses required

Formula to determine number of useable
hosts

2
n
-2

2
n
(where n is the number the number of host
bits remaining) is used to calculate the number
of hosts

-2 Subnetwork ID and broadcast address
cannot be used on each subnet

Subnetting Example Based
on Host Requirements

JoyCo Corp. has been allocated IP network 130.88.0.0/16
and is split into departments that may have up to 1000
computers each.

How many equal-sized subnets can they create with at least 1000
valid host addresses in each?

What subnet mask should they use?

What are the resulting Subnet IDs?
TDC 594

JoyCo Example

Subnet sizes must be powers of 2. The smallest power of
2 that is greater than 1000 is 2
10
= 1024. This would give
them 1024 – 2 = 1022 valid host addresses in each
subnet. Thus, each IP address needs 10 Host bits.

The new prefix length will be 32 – 10 = / 22 .

So, new Subnet mask is 255.255.252.0 .

The difference between old prefix length (16) and new
prefix length (22) is 22-16 = 6 , so 6 bits are borrowed:

Number of new subnets = 2
6
= 64 subnets

We also could have figured this as 65,536 / 1024 = 64
TDC 594

Calculating JoyCo Subnets

First Subnet ID will be the original network address with
the new prefix length.

First Subnet ID is 130.88.0.0 / 22

New Subnet Mask = 255.255.252.0

Two ways to calculate other subnet IDs:

Start with first subnet ID and add 1024 IP addresses to it. This
will increase value in 3
rd
byte by 1024 / 256 = 4 .