Practical Subnetting 5

[Ashely Wolfgram]

The link above also includes a link and description for each video. Making it an easy to remember, one stop-shop for all your IPv4 subnetting needs, and making it perfect for sharing with your colleagues.

Can you please help me find out what I'm doing wrong? I am trying to find out why I can't put that second workstation (192.168.0.129) on the second subnet and access the internet from it or ping the router.

I don't think you can properly do what you are trying to with your current router (unless you can upgrade it to OpenWRT or equivalent.). Its also A LOT harder to do then you think - and probably can't be done through the web interface alone.

Even if you do achieve the above, you are still only part way to your goals.If you are using DHCP, you need to have the DHCP server answer on both subnets, and provide IP's in the appropriate range for each subnet. Again, this is doable but probably not with your current router.

The question to ask though is "Why are you doing this". Doing this does not buy you any significant security/isolation because the systems are still on the same segment, ie computers in one half can read and respond to broadcast traffic in the other half. The typical way of handling this problem is thus a bit more complex - and again, you need more powerful router software to pull it off. (In order to fully understand what I'm going on about here, you need to understand the difference between a subnet and network segment - the 2 concepts go hand-in-hand, and generally 1 subnet=1 segment, but you are describing 2 subnets on 1 segment - which is often not what you want)

The way I have done something similar is thus -I got a router which supported OpenWRT. I configured the LAN ports on the router into different VLANS. (Most 4 port routers are interesting in as much as the 4 lan ports are actually individually accessed, and the software makes them appear as a switch and interchangeable - but you can actually program them to be on different VLANS, and provide per-port isolation). You then put each VLAN in a different subnet, and assign an IP address to the router interface for each subnet. You will probably need 2 switches (if you have more then 3 devices in any subnet) - you would need to use 1 of the switches for each subnet. In this way computers are in different network segments and - from a practical POV - can't talk to each other without going through the router. [ That said, I can point you to articles which say don't rely on VLANS for security - although I don't agree with their conclusions ]

I'm really sorry to do this but I just answered my own question by setting up a static route. I just pretty much followed these directions but tweaked a little: -wrt.com/wiki/index.php/Linking_Subnets_with_Static_Routes

Edit: The static route is completely unnecessary for a minimal setup for mutually exclusive subnets. Just make sure to power cycle the routers after configuring them if they don't work. Here's how I found out: -article?articleNum=132275#b

While it's nice to be able to go through a procedure to calculate all the different numbers in a subnetting problem, it doesn't really help us to know why we need to do subnetting. There are several reasons why we need to subnet. One is to keep our networks to a manageable size to improve performance, manageability and security. Another reason is to preserve a limited supply of IP addresses. Is there a limit to how small we want our networks? Yes. Every time we subnet and create another network, we need a router or layer 3 switch to connect it to the other networks. So, one of the limiting factors is hardware cost. With that in mind, lets see if we can apply what we've learned about subnetting to a more practical problem.

We have two groups of devices separated by two routers. (See Figure 1.) We need to subnet our new class C to provide three subnetworks. The first will be used for the PC0 group which has 8 devices, the second will be used for the PC1 group which has 9 devices but there is an expansion project being planned which will add another 4 devices, and the third network will be used to connect the two routers together.

Since we're starting with a class C, the default mask is 255 . 255 . 255 . 0. We need three subnetworks. We could start by calculating the number of bits we need to borrow from the host portion to create three subnetworks. Three is not an even power of two. The next higher power of two is four or 22. That means we need to borrow two bits making a mask of /26. While that would create the required three networks, it would be wasteful because it would create three networks with 26 - 2 = 62 valid host addresses on each subnetwork. Remember, we only need a maximum of 13 valid host addresses on each subnetwork. So, in order to create an efficient subnetworking plan, we should start by calculating the number of HOST bits we need rather than the number of SUBNETWORK bits we need to borrow.

Ok, we need a maximum of 13 valid host addresses per subnetwork. Don't forget to account for the two extra addresses for the network and broadcast. That means we really need 15 total IP addresses per subnetwork. We'll use the same technique as above. First, 15 is not an even power of two. The next higher power of two is 16 or 24. That means we need 4 bits for the HOST portion. That leaves 4 bits for the subnetworking portion. That makes our new subnet mask

Can you see how this is more efficient than simply borrowing 2 bits from the host portion to create a mask of /26? With a /26, we can address all of our devices and keep the three subnetworks separated but we would be using the entire class C that was assigned to us. With our efficient plan, we created 16 subnetworks and only used three leaving 13 subnetworks for future use.

Now we can assign IP addresses, subnet masks and default gateways to all of our devices. Let's use the 10.0 subnetwork for the PC0 subnetwork, the 10.16 subnetwork for the PC1 subnetwork and the 10.32 subnetwork to interconnect the two routers.

With everything configured as above, we can ping from PC0 to both interfaces on Router 0 and from PC1 to both interfaces on Router 1. If we configure routes on the routers, we'll be able to ping all the way across from either PC to the other PC. Everything works and our plan is complete.

So, let's make the plan even more efficient by taking one or our new subnetworks and subnetting it again to create a network with only two IP addresses. Let's use the last subnetwork, 192 . 168 . 10 . 240. Because we need only two hosts (four total IP addresses) we only need 2 HOST bits. That makes our mask:

Looking at the subnetwork bits, we can see that the first one from the right is in the 4's place. That makes our increment 4. Starting at 192 . 16 . 10 . 240, we count by fours to get all of our new subnetwork numbers.

That completes our subnetting plan. We have fulfilled all the requirements and we have 13 subnetworks with 14 valid hosts each and three subnetworks with two valid hosts each left over for future use. (I had trouble making the Packet Tracer simulation work as shown using RIP. It works with static routes. Mr. Clauss informed me that I had to configure RIP ver 2 on each router to get support for subnetted networks. Once I did that, it worked fine. Thanks to David Clauss from CNM.)

Networking technologies like VLANs (Virtual Local Area Networks) and subnets are often perceived as complex, yet they play a critical role in efficient network management. This guide aims to demystify these concepts, particularly focusing on their application in medium-sized business environments.

Subnetting is a fundamental technique used in dividing a larger network into smaller, more manageable segments called subnets. Each subnet functions as a mini-network, which can be managed and secured independently. This segmentation leads to improved network performance, better management, and enhanced security.

Imagine a corporate office with several departments: Sales, HR, IT, and Customer Support. Each department has specific network requirements and traffic patterns. By using subnetting, the network administrator can create a tailored network environment for each department.

A university campus network supports a variety of users and devices, including faculty, students, administrative staff, and IoT devices. Subnetting is used to segment the network based on usage patterns and security requirements.

In conclusion, subnetting is a strategic tool that enhances the overall functionality and security of a network. By providing a means to organize and manage networks logically, subnetting becomes an essential component in the network architecture of various environments, from corporate offices to educational campuses.

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In summary, VLANs are essential in modern network design, offering a versatile and cost-effective solution for creating segmented, secure, and efficient networks in various organizational environments.

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VLANs and subnets, when understood and applied effectively, can transform a cluttered, vulnerable network into a streamlined, secure, and efficient system. For medium-sized businesses, these technologies are not just optional; they are essential tools for sustainable growth and security in the digital age.

A VLAN (Virtual Local Area Network) is a logical division of a network at the data link layer, creating separate broadcast domains within the same physical network. It helps in segmenting network traffic based on functional, departmental, or security needs. A subnet, or subnetwork, on the other hand, is a logical division of an IP network at the network layer. It breaks down a larger network into smaller, manageable parts, each with its own unique IP address range.

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