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Setting Up Subnetworks

written by: Jesma•edited by: J. F. Amprimoz•updated: 3/31/2009

Understanding how subnetworking works doesn't necessarily mean that you can put the knowledge to use. In this article we explain how to calculate subnet host address ranges and implement them on a real network.

  • slide 1 of 3

    Network and Broadcast Numbers

    On every network and subnetwork, there are two reserved IP addresses. One of these belongs to the network, and the other to the broadcast. What does that even mean?

    The network address is reserved as it is used to define the network as a whole. You couldn't have a workstation with the IP address 10.0.0.0 because that address is reserved for referring to the network. You'd have to start with 10.0.0.1 and work from there.

    The broadcast address is what is used to communicate, simultaneously, with all nodes on the network. On our "10" network, the broadcast address would be 10.255.255.255.

    These same rules apply even when you subnetwork. Every subnetwork still needs to have one network address and one broadcast address. This means that the more you break down a single IP, like 10.0.0.0, the less hosts it can support. You lose two host addresses for every subnet you add. This isn't really a problem with Class A networks, but can become a serious issue with Class C networks.

  • slide 2 of 3

    Defining Address Ranges

    Let's apply all this information to our subnetworking now. In the previous article, we subnetworked the 10.0.0.0 address by changing the subnet mask by 1 bit, to 255.128.0.0. Now we have the task of figuring out host address ranges within each subnetwork. Defining ranges is, fortunately, much more intuitive than the actual act of subnetting. If you divided 10.0.0.0 into two, it would make sense for the first range to extend halfway through the available host addresses and the second to extend from the halfway point to the end, right? That's exactly how it is. Therefore, for two subnetworks with the subnet mask of 255.128.0.0 we come up with the following:

    Subnet 1

    Network: 10.0.0.0

    Broadcast: 10.127.255.255

    Hosts: 10.0.0.1 - 10.127.255.254

    Subnet 2

    Network: 10.128.0.0

    Broadcast: 10.128.255.255

    Hosts: 10.128.0.1 - 10.255.255.254

    One of the best ways to perform any kind of subnetting calculations is to use a specially designed subnet calculator. These are freely available online, and most can perform calculations automatically depending on your selections, which can be: Number of hosts per network, number of total networks, and more.

  • slide 3 of 3

    How Network Nodes Should Be Configured

    Now that you have figured out the subnet mask and host address ranges for your subnetworks, you can implement the changes on your network devices.

    There is actually very little that needs to be changed. You will need a router between the two networks. The easiest way to implement subnets is to configure one of the router interfaces with an IP address from Network 1, like 10.0.0.1, and configure a second interface with an IP address from Network 2, like 10.128.0.1. To simplify the task of assigning IP addresses to each node on the network, you can configure a DHCP-enabled router to assign addresses in the Network 1 range to one "side", and Network 2 addresses to the other. Alternatively, you can do DHCP from a server on each subnet, configured much the same way. While there is a way to do DHCP across multiple subnets from one DHCP server located on one subnet, that involves getting into far more complex subjects than simple subnetting.

    That concludes our introductory look at subnetworking. I hope that now you have a clear understanding of what subnetworking is? Why it is worth doing? How it works? And how its done?

Complete Guide to Subnetworking

Learn all about what subnetworking is, why subnets are used, how to calculate subnet masks, and how to define subnet IP address ranges. We do more than provide you with the answer like a subnet calculator does - we teach you how to figure it out for yourself.
  1. Introduction to Subnetworking
  2. How to Subnetwork - Calculating Subnets
  3. Setting Up Subnetworks