# Help: How to subnetting

Nov 15th, 2011

Good morning:

I know make subnetting when class A, B and C and his mask are respected eg. 192.168.1.100 /26 but I don't know when e.g. I'd to subnet clasless e.g. 136.170.96.24 /29. It's an ip of class B but the subnet mask not.

Can anyone help me?. Thanks.

Overall Rating: 5 (2 ratings)

## Replies

Ven Taylor Wed, 11/16/2011 - 11:55

Ignore the fact that it's a class B when you're dealing with numbers higher than /24.

anything with /29 is a very small subnet (duh) and 136.170.96.24 subnet would be 136.170.96.25 - 136.170.96.30 with .31 as a broadcast.

/25 subnets increase by a factor of 128.

/26 subnets increase by a factor of 64.

/27 subnets increase by a factor of 32.

/28 subnets increase by a factor of 16.

/29 subnets increase by a factor of 8.

/30 subnets increase by a factor of 4.

The math keeps working regardless of how low you go with the numbers.

Make sense??

Ven

reg_cisco Tue, 11/22/2011 - 11:27

I don't understand anything of what you are answering me.

I think I'd start to subnet 192.168.30.0 /26 to have a simple example, can anybody help me?.

Ven Taylor Wed, 11/23/2011 - 12:05

192.168.30.0/26 gives you 192.168.30.1 - 192.168.30.62 with 192.168.30.63 as a broadcast address.

An IP address is 32 binary bits.  /26 just means "use 26 binary bits as the subnet mask".

/25 subnets have 128 addresses (.1-.126 with 127 as broadcast)  half of a /24

This gives you a .0 subnet and a .128 subnet.

/26 subnets have 64 addresses (.1-.62 with .63 as a broadcast)    1/4 of a /24

This gives you a .0 subnet, a .64, a .128, and a .192 subnet

Make sense?

Ven

Oleksandr Nesterov Thu, 12/29/2011 - 03:07

Hi

192.168.30.0 /26

You know that address that starts from 192 is in class C and standard mask for it is 24. Address mask always describes which part of address describes network (26 is a number of bits in IP address that describes network portion).  Also you know that for class C first 3 octets describe network and last octet describes a host.

Each octet has 8 bits. Since we have mask 24, 24/8 = 3 - octets describes network - 192.168.30

The last octet describes host and for it we have 8 bits. But in your example we have mask 26 - so seems like we will have 26 bits that describes network 24 of them are turned into  192.168.30 and two of them we have to take from last octet:

192.168.30.(00)000000 - first 3 octets are in decimal form and last one int binary. And in the brackets network address shown - subnetwork

If we will change 2 bits in the brackets we will get following addresses:

192.168.30.(00)000000 - 192.168.30.0

192.168.30.(01)000000 - 192.168.30.64

192.168.30.(10)000000 - 192.168.30.128

192.168.30.(11)000000 - 192.168.30.192

These are subnetworks within the network 192.168.30.  - and here class separation is not applicable.

And within this subnetworks you will have addresses for hosts (6 bits left for hosts). For example:

In the network 192.168.30.0 host addresses will be:

192.168.30.(00)000001 - 192.168.30.1  =  192.168.30.(0) + 1

192.168.30.(00)000010 - 192.168.30.2  =  192.168.30.(0) + 2

192.168.30.(00)000011 - 192.168.30.3  =  192.168.30.(0) + 3

192.168.30.(00)000100 - 192.168.30.4  =  192.168.30.(0) + 4

192.168.30.(00)000101 - 192.168.30.5  =  192.168.30.(0) + 5

192.168.30.(00)000111 - 192.168.30.6  =  192.168.30.(0) + 6

……

192.168.30.(00)111110 - 192.168.30.62  =  192.168.30.(0) + 62

192.168.30.(00)111111 - 192.168.30.63  =  192.168.30.(0) + 63   - broadcast

In the network 192.168.30.128 host addresses will be

192.168.30.(10)000001 - 192.168.30.129  =  192.168.30.(128) + 1

192.168.30.(10)000010 - 192.168.30.130 = 192.168.30.(128) + 2

192.168.30.(10)000011 - 192.168.30.131 = 192.168.30.(128) + 3

192.168.30.(10)000100 - 192.168.30.132 = 192.168.30.(128) + 4

……

192.168.30.(10)111110 - 192.168.30.190 = 192.168.30.(128) + 62

192.168.30.(10)111111 - 192.168.30.191 = 192.168.30.(128) + 63 - broadcast

Also you probably know that address where host portion is all-zero is a network address , and address with all-ones in host portion is used for broadcast.

NOTE:

Important thing - you MUST be familiar with binary mathematics - it will follow you in ALL network technologies.

alan.deery Mon, 02/13/2012 - 04:57

The company I work with is running a free seminar on IP Addressing and Subnetting, 28th of Feb 2012 3pm GMT:

Objectives: at the end of this online seminar participants will be able to understand how IP addresses work and how to split a big network into smaller networks (subnet). A full understanding of this subject is required for anyone who is sitting for Network+, Windows 7, Windows 2008 Network infrastructure and CCNA exam.

Seminar outline:

IP Class A, B, C and D

2 – Understanding IP Packets
Unicast
Multicast

3 – Understanding IP Subnet
What is subnet for?
Working with 2 subnets
Working with 4 subnets
Working with 6 subnets

Cisco_Learning_... Wed, 02/22/2012 - 12:00

Friendly Reminder: Effective this Friday, February 24th, at 5:00 p.m. (PT) all new discussion thread requests will be redirected to the Cisco Learning Network.  We encourage you now to go there to create any new discussion thread or document regarding Certifications and/or Training topics.