What are we going to do? Single Subnet Size Maybe, two additional sites will be added sometime in the future. They expect that the number of VLANs and users could double. Let’s say we get the following answers to our questions: What is the expected growth of this network?.How many hosts do we have in each VLAN?.How many VLANs are there? Each VLAN requires a different subnet.Let’s look at the network we used earlier:Ĭan we say anything about the number of subnets that we need here? Not really, the only thing we can (probably) tell is that we need a subnet in between each router pair. Let’s look at an example so that I can explain the choices we have to make when designing a network. These calculations can be done with subnet calculators (you can find dozens with a google search) but on (Cisco) exams, you have to calculate these on your own. In between the routers, you might want to use a smaller subnet with only enough IP addresses for the two routers. Calculations: Above, I used a /24 subnet mask which is easy but not efficient.For example, if we add R5…what subnet are we going to use? What if we create multiple VLANs on SW1? What subnets will you use for these VLANs? You need to think about this beforehand or things will get messy. Creating a design: like the examples above, you have to think of how many subnets you need and which ones you are going to use.When we talk about subnetting, there’s two things we have to do: We are still wasting a lot of IP addresses though with the links in between our routers so there’s room for improvement. You can get a lot of subnets out of that… This network offers so much more space, the private range starts with 172.16.0.0 and ends with 172.31.255.255. We are now using subnets from the class B 172.16.0.0/16 network. We could improve this network somewhat by using subnets from the class A or B range. Public IP addresses are very limited so you’ll have to use smaller subnets so that you don’t waste any. This is no problem when we use private IP addresses but it is an issue when we use public IP addresses. When we use a /24 subnet mask, we are wasting 252 IP addresses. The links in between the routers are point-to-point links so we only need two IP addresses, one for each router.What if we have more than 256 sites? There’s not enough space so you will have to pick another network range. There’s a limited amount of class C (private) networks.Technically, this will work but why is using these networks a poor choice? There are two reasons: ![]() We are using the following class C networks: Behind each router, we find a switch with a host. ![]() Each router represents a site for this company. Why would you want to do this? Let me show you a picture:Ībove we have a small company network with four routers. This subnet falls within the 172.16.0.0/16 class B network so that’s why it is called a “sub”net. Instead of one big network, we can use a smaller “portion”. Remember our class A, B and C networks?Ī subnet is a sub-network of a network that falls within the class A, B or C range.įor example, 172.16.0.0/16 is a class B network. ![]() In this lesson, we’ll take a look at what subnetting is and why we use it.
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