The best way of learning subnetting is to do it. Here are a selection of worked examples to help you get started.

At the end are some links to online quizes so you can do it yourself.

**29.0.0.0**. You need to create at least 20 networks and each network will support a maximum of 160 hosts. Would the following two subnet masks Work?

**255.255.0.0** and or **255.255.255.0**

Yes both would work.

Mask **255.255.0.0** has 8 bits for the subnet and 16 bits for the host

8 bits would accommodate 2^{8}=256 subnets

16 bits would accommodate 2^{16}= over 64000 hosts

Mask 255.255.255.0 has 16 bits for the subnet and 8 bits of the host.

Have possible 2^{8} -2 hosts =254 which is enough

Easiest is to sub net on a byte boundary which would mean a subnet mask of **255.255.255.0**

This would allocate **8 bits** for the **subnet** and** 8 bits** for the **host.**

We need to accommodate around 200 hosts which requires 8 bits which we have.

We need 4 subnets which require 4 bits and we have 8 bits. So we have more than enough.

Decimal 192 =11000000 binary which means that 2 bits of this octet are used for the subnet. Now add the 24 bits 255.255.255 and we have 26 bits. So we write:

222.1.1.20/26

Decimal 248 =11111000 binary which means that 5 bits of this octet are used for the subnet. Now add the 16 bits 255.255. and we have 21 bits. So we write:

135..1.1.25/21

A class C address has 8 bits of the host which will give 2^{8} -2 =254 hosts

Current mask= 255.255.255.0

Bits needs for 10 subnets =4 =2^{4} =16 possible subnets

Bits needs for 12 hosts = 4 = 2^{4} = 16-2=14 possible hosts.

So our mask in binary =**11110000**= **240** decimal

Final Mask =**255.255.255.240**

What is the subnet mask for the maximum number of hosts?

How many hosts can each subnet have?

What is the IP address of host 3 on subnet 2 ?

Current mask= 255.255.255.0

Bits needs for 30 subnets =5 =2^{5} =32 possible subnets

Bits left for hosts = 3 = 2^{3} = 8-2=6 possible hosts.

So our mask in binary =**11111000**= **248** decimal

Final Mask =**255.255.255.248**

Address of host 3 on subnet 2 is

subnet 2 =00010000 host 3 =000000011

Add the two together =00010011=19

therefore IP address of host 3 on subnet 2 =205.11.2.19

What are the two possible subnet masks?

Current mask= 255.255.255.0

Bits needs for 48 hosts = 6 = 2^{6} = 64-2=62 possible hosts.

Bits needs for 2 subnets =1 =2^{1} =2 possible subnets

Total of 7 bits needed so therefore we can use either 1 bit or 2 bits for the subnet. So we could have

1 bit subnet 7 bits hosts or 2 bits subnet 6 bit host

masks are 10000000 and 11000000 =128 decimal and 192 decimal.

Final possible masks are:

**255.255.255.128** and **255.255.255.192**

**255.255.255.192**What is the host address and subnet of the following IP address 197.1.2.67.

192 in binary =11000000 gives 4 possible subnets of (showing 2 most significant bits):

00,01,10,11

67 in binary =01000011

So Applying Mask:

**Videos**

**Related Tutorials and Resources:**

- binary numbers for beginners tutorial
- IPv4 Addresses and Classes
- Subnets and subnet Masking explained.
- IPv6 Guide

### Subnet Quizzes

Test you knowledge using these online quizzes

You are given 2 networks to be implemented on the following topology:

Network 1 172.16.0.0

Network 2 192.168.0.0

You are required to subnet the above mentioned networks as per following requirements

Net1 Subnet1 100 Hosts

Net1 Subnet2 500 Hosts

Net1 Subnet3 200 Hosts

Net2 is subnetted for all router to router links. No hosts other than required to router interfaces are to be assigned. Implement EIGRP with autonomous System no. 1 in each router, assign proper IP address, subnet masks and default gateways where needed plz help me inthsi case

Hi

Sorry but the question requires more time to answer than I can provide. I can only point you to the examples. You might want to go on to the cisco sites as they have more detailed examples focused on the cisco exams

Rgds

Steve

You have been allocated a class b network address of 132.121.0.0 and need to create 150 subnets each with 120 hosts , what is the new subnet mask to satisfy these criteria?

new subnet mask:

host per network: 2^n-2 ( n is the zeroes )

class B is 255.255.0.0 1111 1111. 1111 1111 . 0000 0000 . 0000 0000

I need to steal hosts bits (the zeroes) to get close or around to 150 : so that would be 2^6=64 or 2^7=128?

Im stuck on finding the new subnet mask please help me

150 subnet means you need 8 bits which is ok as you have 16. so use a class C mask

255.255.255.0

this give 254 subnet and 254 hosts

A router has received a packet with destination IP address 192.168.67.10. The subnet mask added by network administrator in the router routing table is 255.255.224.0. How the router will find the subnet address(ID)?

By a logical AND between the IP address and the subnet mask

For question #7: Shouldn’t the answer be 205.11.2.9

i dont understand why 19 when you’re going up by 6 users starting from 0.

Hi

The answer is correct

205.11.2.9 would be host 1 on subnet 1

rgds

steve

please who can solve this problem for me?????

You are given an IP address from your ISP:

220.42.56.20

Design a network having 4 subnets for

Administrative staff,

Academic staff and

Postgraduate students,

Undergraduate students

Looks like the question is wrong are you sure of the IP address

Hello Steve, your question are very helpful ..but can you guide me on the below ones please

1.You currently use the default mask for your IP network 192.168.1.0. You need to subnet

your network so that you get the following.

a. 30 additional networks, and 4 hosts per network.

b. 20 hosts per subnet

c. Only 4 additional networks

Hi

4 hosts means you need 3 bits for the host so your mask looks like this

11111000

20 hosts means that you need 5 bits for the host so your mask looks like this

11100000

4 additional networks means you need 3 bits for the Network so your mask looks like this

11100000

I’ll let you convert them to decimal

Rgds

Steve

Great job steve!

All your answers are 100% correct and I found the exercises very helpful. Thanks again!

what is the last valid host on the subnetwork 172.21.80.128/26?

pls help me in this

thanks a lot

Hi

you have 2 bits for the host so values are

=00,01,10,11

00 and 11 are not usable so you have hosts 01 and 10

by last I assume the mean 10 so the address is 172.21.80.130

CIDR is /26

172.21.80.128

Binary: 10101100 00010101 01010000 10000000

Let’s seperate the subnet and hosts

10101100 00010101 01010000 10 —> 000000

Lets make all the 0’s 1’s where the seperation is to get Broadcast

10101100 00010101 01010000 10 —> 111111

Broadcast : 172.21.80.191 (191 is 10 111 111)

Last usuable host IP will therefore be 10 111110

Last usuable IP: 172.21.80.190

Q.6 . Subnet 10 host 1 ip address must be 195.1.1.151 as per my calculation

Hi

Just checked it again and 161 is correct

rgds

steve

1. Calculate IP addresses and subnet mask for 3 nodes (indexes are listed into the 3rd line) for three subnets (indexes are listed in the 2nd line). Network can contain some amount of subnets and no less than it is specified (the 1st line, S). Each subnet can contain no less than specific numbers of nodes (the 1st line, H).

Result: final nodes IP and masks with calculaations

300, 3, 140

1, 2, 3

11, 12, 13 / 1, 22, 23

The 1st line: N, S, H; N – total nodes amount, S – subnet amount, H – subnet nodesamount.

The 2nd line: A, B, C; A & B & C – subnet indexes

The 3rd line: a1, a2, a3 / b1, b2, b3; ai и bi – node indexes to calculate. ci is equal to bi

Help please if someone knows

Sorry find the question confusing were did it come from

Rgds

Steve

Question one should be a MINIMUM of 160 hosts, not a MAXIMUM!

No Maximum is correct.But using Minimum would also work in this case.

Rgds

Steve

give the following subnets ,represnt them with a single network address

212.56.120,0

212.56.121.0

212.56.122.0

212.56.123.0

Not sure about the question where is it from

Rgds

Steve

Amazing, amazing work! You have no idea how much these exercises have helped, and the explanations are all very clear as well. Thank you very much!

p.s. I love the font

Hi Steve, thanks for producing this tutorial.

In question 9 – is this assuming a Class C network? E.g. if it were a Class B network with subnet mask of 255.255.255.192 then there would be 10 bits for the subnets rather than 2.

Yes it is a class C address

Four valid subnets are to be created in a class B IP address 129.22.0.0.

Calculate the required Subnet mask and fill in the table below.

The answer “therefore IP address of host 3 on subnet 2 =205.11.2.19” of question 7 is wrong and by the way the question is wrong, because if you calculate the ip address of 2nd host pf 3rd subnet then you will get the answer as 205.11.2.19…….

And if we go according to question the real ans should be 204.11.2.11……

Correct me if I’m wrong!

The answer is correct.

The first sub network address is 00001xxx =decimal 8 and the second is 00010xxx =decimal 16

rgds

steve

is there any one who can explain the 9th example? i am not getting that ans .

What answer do you get

rgds

steve

Very useful exercises 😉 but I have a question about exercise 6: Shouldn’t it be 255.255.255.248 instead of 255.255.255.240? Because if we choose the last one we will get more than the 12 hosts.

Hi

the answer is correct as 248 would only give us 8 hosts (3 bits) and we need at least 12.

Yes but since it is typed “10 subnets each with a maximum 12 hosts on each subnet.” and 8 is smaller than 12 I assume 255.255.255.248 is the correct answer.

Maximum of 12 means that you can have 12. But with 8 bits you can only have 8.

Does that make sense

rgds

steve