What is Subnetting?

Here you will find subnet network overview, ip addressing, address translation, network overview, subnet masking and subnetting overview. A subnet or a subnetwork is a separate part of an organization’s network. In a subnet all the machines are typically in one room, building or at one geographical location.


By dividing an organization’s network into the subnets allows it to connect to the internet by using the same shared network address. Without subnet’s an organization may get different connections to access the internet. Subnetting is the modification of a single IP network to create two or more logically different networks.

A subnet allows the flow of network traffic between hosts to be segregated based on the configuration of a network. Subnetting can improve the network security and performance by arranging the hosts into the different logical groups. Subnetting is required when one network address needs to be distributed across multiple network segments. Subnetting is required when a company uses two or more types of the network technologies like Ethernet and Token Ring.


Two network segments are restricted by distance limitations. Submetting or dividing the network into the segments is also required when localized network management is required for example accounting, sales, customer service departments. There is another reason for the subnetting, which is that the computers on the network, which use more bandwidth, needs to be separated from the rest of the computers. There are certain advantages and disadvantages of the subnetting. Before you start dividing your network into the different segments, you should assign the IP address to each computer in your network segment.

Subnetting makes the network management easier and it is also very helpful for the troubleshooting of a network segment. The internet is a collection of networks where users communication with each other. Each communication on the internet carries the source and the destination address of the computer. This address is called IP address. This 32 bit address has two parts: one part represents the network portion and the other part represents the host portion of the IP address. A company can use some of the bits in the machine or host portion of the address to identify a subnet. In this scenario, the IP address contains three parts: the network address, the subnet address and the machine address.

Subnet Mask Basics

The most recognizable part aspect of subnetting is the Subnet mask. A subnet mask contains 4 bytes, 32 bits and is divided into 4 period separated octets. Typically, a very common subnet mask in binary looks like this.

11111111 11111111 00000000 00000000
255 255 0 0


How to Apply a Subnet Mask

A subnet mask does not work like an IP address and it cannot exist separately without an IP address. An IP address and subnet mask work together to form a network. An IP address splits into two main parts when applying the subnet mask. The leftmost bits of a subnet mask must be set to 1. For example

11111111.00000000.00000000.00000000
11111111.11111111.00000000.00000000
11111111.11111111.11111111.00000000

The above example shows the valid representation of a subnet mask into the binary numbers.

00000000.00000000.00000000.00000000 is an invalid subnet mask.
11111111.11111111.11111111.11111111 is also invalid subnet mask.

All valid subnet masks contain two parts: the left side with all mask bits set to '1' (the extended network portion) and the right side with all bits set to '0' (the host portion), such as the first example above.

Subnetting an IP network can be performed for a variety of reasons such as using the different physical media in an organization, such as FDDI, WAN and Ethernet, preservation of the addresses and for the purpose of security, management and ease of troubleshooting. The most

common reason of the subnetting is to control the network traffic. In an Ethernet network, all computers in a segment see all the packets that are transmitted by all the other computers on the same segment.

In this situation, the network performance can be badly affected due to the heavy traffic loads, collisions and the retransmission of the packets. A router is used to connect the IP networks and it also helps to minimize the load of the traffic.

Subnet Masking

By applying the subnet mask to the IP address you can identify the network and host portion of the IP address. The decimal number 1 represents the network portion in the subnet mask and the node is represented the 0s. Performing a logical AND operation between the IP address and the subnet mask resulting in the network address.

For example, using our test IP address and the default Class B subnet mask, we get:
10001100.10110011.11110000.11001000 140.179.240.200 IP address of the class B
11111111.11111111.00000000.00000000 255.255.000.000 Default subnet mask of class B
--------------------------------------------------------
10001100.10110011.00000000.00000000 140.179.000.000 Network Address value

The following example shows the default subnet masks.
• Class A Subnet Mask- 255.0.0.0 - 11111111.00000000.00000000.00000000
• Class B Subnet Mask- 255.255.0.0 - 11111111.11111111.00000000.00000000
• Class C Subnet Mask- 255.255.255.0 - 11111111.11111111.11111111.00000000

Subnetting Review

Subnetting allows network and system administrators some flexibility in defining relationship among the hosts of a network. Hosts on the logically and physically different subnets can talk to each other through specialized devices called gateway or router. The ability to filter the traffic between

subnets can make the more bandwidth availability. Subnetting referred to as subdivision of a class based networks into subnetworks.

A router can exchange subnet routes with the other routers in the network. A subnetted network can’t be split into the isolated portion. All the subnets must be contiguous because the routing information cannot be passed to a non-network member. Router can exchange subnet routes with other routers within the network. Since the subnet masks are identical across the network, the routers will interpret these routes in the same manner. However, routers not attached to the subnetted network can't interpret these subnet routes, since they lack the subnet mask.

Therefore, subnet routes are not relayed to routers on other networks. This leads to our second

restriction. Subnetting allows you to create multiple logically different networks within the same class A, B or C. If you break a major network into smaller networks, it allows you to create a network of interconnecting subnetworks. Any device or gateway that is responsible for connecting the different subnetworks must have the distinct IP address one for each subnetwork.

To subnet a network use and extend the natural subnet mask using some of the bits from the host ID portion to create a subnetwork ID. In this example, given a Class C network of the IP address 4.15.5.0 which has a natural subnet mask of 255.255.255.0, you can create subnets in this manner:

11001100.00001111.00000101.00000000 204.15.5.0
11111111.11111111.11111111.11100000 255.255.255.224

---------------------------------|subnet|----

By extending the natural subnet mask to be 255.255.255.224, you have used three bits from the host portion of the mask and used them to make subnets. By using these 3 bits, it is possible to create 8 subnets. The remaining five ID bits of the host portion, each subnet can make 32 host addresses and the 30 addresses out of 32 are assigned to the devices or computers. The host IDs

of all zeros and all ones are not allowed.

204.15.5.0 255.255.255.224 host address range 1 to 30
204.15.5.32 255.255.255.224 host address range 33 to 62
204.15.5.64 255.255.255.224 host address range 65 to 94
204.15.5.96 255.255.255.224 host address range 97 to 126
204.15.5.128 255.255.255.224 host address range 129 to 158
204.15.5.160 255.255.255.224 host address range 161 to 190
204.15.5.192 255.255.255.224 host address range 193 to 222
204.15.5.224 255.255.255.224 host address range 225 to 254

Introduction to Network Security

You will find here network security overview, networking solutions, anti virus, anti spamming tips, trojan horses, malware, adware. Security is an essential element in maintaining any network. The main focus of the IT managers and computer network administrators is to secure the computer networks.

Users are happy to get the data on time and without any problem. The use of the authentication and biometrics can improve the security to some extend. Computer Security in the data communication cannot be compromised. Hackers’ can exploit the sensitive and financial and corporate data. There can be many threats to a computer network and it’s the responsibility of a computer network administrator to keep the computer network secure from these threats. 

IT Administrator should know about all the possible security attacks and also know their solutions. IT Administrator should look for the viruses from the Internet, Malware, Adware, Trojan horses, E-mail attachments, Floppy disk, CD or any infected computer that is attached to the network. Spyware and network intrusions are specifically designed to get the secret information from their target companies, which can do harm for the company. Everyday security threats are refined as hackers designed new security threats.

The main cause of a security threat in the small companies is the misuse of the internet without proper anti virus, anti spyware installed on the every PC of the company’s network. For example if a company’s employees browse an inappropriate website, sends or receives the infected data, leaks company’s secret information, then there are greater chances for a possible virus/malware attack.

In my opinion, the end user’s education on the security threats, preventions and precautionary measures are must. They should be trained about the possible and easy virus attacks from the internet if they browse the inappropriate sites.

Another important thing is the insider’s attack e.g if the company’s employee leave the company for any reason, then its very important that all the computer and other company’s sensitive assets access should be revoked by him immediately. I have personally seen a situation, when a company’s network administrator was dismissed and he access the company’s server by VPN and send harmful viruses to the server and the entire computer network, and it was impossible for the IT manager to control all the security attacks in one time.

It’s the responsibility to keep a closer eye on the new employee’s activities, their access to the sensitive data, and computers servers. IT managers should bring it in the knowledge of the employees that any change in their computer will be logged. So that nobody can even try to do anything that is not permitted.

There are six basic security steps for the Windows platforms. If a network administrator can follow these steps then he/she can save the computerss from all the possible security threats and virus or malware attacks.

First the IT managers should divide the computer network into the segments. They should filter the access to the internet with the help of a firewall by blocking TCP port 1433 and TCP port 1434. Internet access from the outside should be allowed to the SQL systems. All the unwanted ports should be blocked and only the required ports should be open for access.

Second, moderate the affect of the spoofed ports. The port 80 is the most commonly used port.

Third, as network administrator you should install the current patches to you’re your server computer and client’s up to date. Patches can prevent the systems from the known vulnerabilities. Latest patches can be downloaded from the Windows website. You can also configure to automatically update. Also third party patches products are also available that can be tested and installed. Additionally, by strengthening the user authentication process can be very helpful for security purposes.. You can use password security and other technological methods for the authenticate purpose.

Fourth, you can limit the number of the network administrators it can also be helpful for security a computer network. Admin rights should not be given to the local PC, until and unless it is the requirements for the applications that are installed on the local PCs.

Fifth, protect computers against the known attacks. Don’t disable any Windows known service such as clipbook, Telnet etc. Set the powerful permission the shared network resources.

Last, you can configure the security policies. Implement the security policies on your network.

Security measures and methods have expenses with their purchases. Deployment, maintenance and the implementations of these methods can increase the security cost. Some other things which a IT administrator or a IT manager should keep in mind are the education of the employees about the computer networking, security, use of the encryption and digital right management software, block the unwanted emails and audit security on the regular basis. Network security methods should be used and implemented in order to prevent your computer network from the unauthorized access.

In this article you have discovered the Computer Network Security Overview More topics to come are Cisco Labs, Routing, IP addressing & Free IT resources.