subnet (subnetwork)

A subnet (short for "subnetwork") is an identifiably separate part of an organization's network.

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A subnet (short for "subnetwork") is an identifiably separate part of an organization's network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization's network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnetworks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization.

The Internet is a collection of networks whose users communicate with each other. Each communication carries the address of the source and destination networks and the particular machine within the network associated with the user or host computer at each end. This address is called the IP address (Internet Protocol address). This 32-bit IP address has two parts: one part identifies the network (with the network number) and the other part identifies the specific machine or host within the network (with the host number). An organization can use some of the bits in the machine or host part of the address to identify a specific subnet. Effectively, the IP address then contains three parts: the network number, the subnet number, and the machine number.

The standard procedure for creating and identifying subnets is provided in Internet Request for Comments 950.

The 32-bit IP address is often depicted as a dot address (also called dotted quad notation) - that is, four groups (or quads) of decimal numbers separated by periods. Here's an example:

 130.5.5.25

Each of the decimal numbers represents a string of eight binary digits. Thus, the above IP address really is this string of 0s and 1s:

 10000010.00000101.00000101.00011001

As you can see, we inserted periods between each eight-digit sequence just as we did for the decimal version of the IP address. Obviously, the decimal version of the IP address is easier to read and that's the form most commonly used.

Some portion of the IP address represents the network number or address and some portion represents the local machine address (also known as the host number or address). IP addresses can be one of several classes, each determining how many bits represent the network number and how many represent the host number. The most common class used by large organizations (Class B) allows 16 bits for the network number and 16 for the host number. Using the above example, here's how the IP address is divided:

 

 <--Network address--><--Host address--> 130.5 . 5.25

If you wanted to add subnetting to this address, then some portion (in this example, eight bits) of the host address could be used for a subnet address. Thus:

 <--Network address--><--Subnet address--><--Host address--> 130.5 . 5 . 25

To simplify this explanation, we've divided the subnet into a neat eight bits but an organization could choose some other scheme using only part of the third quad or even part of the fourth quad.

Once a packet has arrived at an organization's gateway or connection point with its unique network number, it can be routed within the organization's internal gateways using the subnet number. The router knows which bits to look at (and which not to look at) by looking at a subnet mask, which is a screen of numbers that tells you which numbers to look at underneath. In a binary mask, a "1" over a number says "Look at the number underneath"; a "0" says "Don't look." Using a mask saves the router having to handle the entire 32 bit address; it can simply look at the bits selected by the mask.

 

Getting started with subnets
To explore how subnets are used in the enterprise, here are some additional resources:
How to subnet: Subnetting calculations and shortcuts: Whether you need a subnetting cheat sheet or you're preparing for your CCNA exam, this tip and quiz on subnetting shortcuts shows you how to calculate a subnet mask, breaks down IP address classes, and explains binary and how to get bit values of one octet. If you're wondering how to figure out what subnet mask to use when you need "x" hosts and "x" networks, this tutorial shows you.
IP addressing and subnetting: What network administrators need to know: IP addressing and subnetting are an important part of networks. Learn about how IP addresses work, subnet masks, classful vs. classless IP addresses, default gateways and more.
Calculating subnets for IP addresses in IPv6 : Understand how subnetting in IPv6 differs from that of IPv4 and what you need to do in order to calculate the subnet of an IP address in IP version 6.
This was first published in March 2007

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