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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


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 IP address of the class B
11111111.11111111.00000000.00000000 Default subnet mask of class B
10001100.10110011.00000000.00000000 Network Address value

The following example shows the default subnet masks.
• Class A Subnet Mask- - 11111111.00000000.00000000.00000000
• Class B Subnet Mask- - 11111111.11111111.00000000.00000000
• Class C Subnet Mask- - 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 which has a natural subnet mask of, you can create subnets in this manner:



By extending the natural subnet mask to be, 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. host address range 1 to 30 host address range 33 to 62 host address range 65 to 94 host address range 97 to 126 host address range 129 to 158 host address range 161 to 190 host address range 193 to 222 host address range 225 to 254



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