How many subnets are created

It is possible that this was a deliberate design accounting for future growth, but in many cases this is just wasted address space due to the fact that the same subnet mask is used for all the subnets. Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given:. The easiest way to assign the subnets is to assign the largest first. For example, you can assign in this manner:. Figure 5 illustrates how using VLSM helped save more than half of the address space.

Classless Interdomain Routing CIDR was introduced in order to improve both address space utilization and routing scalability in the Internet. It was needed because of the rapid growth of the Internet and growth of the IP routing tables held in the Internet routers.

Length means the number of left-most contiguous mask bits that are set to one. So network CIDR also depicts a more hierarchical Internet architecture, where each domain takes its IP addresses from a higher level. This allows for the summarization of the domains to be done at the higher level. For example, if an ISP owns network Yet, when advertising to other providers, the ISP only needs to advertise A bit subnet mask allows for four IPv4 addresses: two host addresses, one all-zeros network, and one all-ones broadcast address.

A point-to-point link can only have two host addresses. There is no real need to have the broadcast and all-zeros addresses with point-to-point links. A bit subnet mask will allow for exactly two host addresses, and eliminates the broadcast and all-zeros addresses, thus conserving the use of IP addresses to the minimum for point-to-point links.

However, they can also be used on broadcast interface types like ethernet interfaces. If that is the case, make sure there are only two IPv4 addresses needed on that ethernet segment. A subnet mask of These subnets cannot be used for assigning address to network links, because they always need more than one address per link.

The example for Cisco routers is the loopback interface. These interfaces are internal interfaces and do not connect to other devices. Skip to content Skip to search Skip to footer. Available Languages. Download Options. Updated: May 21, Contents Introduction. Introduction This document provides basic information needed in order to configure your router for routing IP, such as how addresses are broken down and how subnetting works. Prerequisites Requirements Cisco recommends that you have a basic understanding of binary and decimal numbers.

Components Used This document is not restricted to specific software and hardware versions. Additional Information If definitions are helpful to you, use these vocabulary terms in order to get you started: Address - The unique number ID assigned to one host or interface in a network.

Subnet - A portion of a network that shares a particular subnet address. Interface - A network connection. Figure 1 In a Class A address, the first octet is the network portion, so the Class A example in Figure 1 has a major network address of 1. Network Masks A network mask helps you know which portion of the address identifies the network and which portion of the address identifies the node.

Class A, B, and C networks have default masks, also known as natural masks, as shown here: Class A: The network subnetting scheme in this section allows for eight subnets, and the network might appear as: Figure 2 Notice that each of the routers in Figure 2 is attached to four subnetworks, one subnetwork is common to both routers.

Examples Sample Exercise 1 Now that you have an understanding of subnetting, put this knowledge to use. DeviceA: Determine the Subnet for DeviceB: Sample Exercise 2 Given the Class C network of Figure 3 Looking at the network shown in Figure 3 , you can see that you are required to create five subnets. An example of how you might assign the subnetworks is: netA: VLSM Example Given the same network and requirements as in Sample Exercise 2 develop a subnetting scheme with the use of VLSM, given: netA: must support 14 hosts netB: must support 28 hosts netC: must support 2 hosts netD: must support 7 hosts netE: must support 28 host Determine what mask allows the required number of hosts.

For example, you can assign in this manner: netB: Special Subnets bit Subnets A bit subnet mask allows for four IPv4 addresses: two host addresses, one all-zeros network, and one all-ones broadcast address.

The mask is Example Example interface Loopback0 ip address Router A hostname routera! S 0 connects to router B router rip network Here is our new subnet mask:.

We need 50 hosts per subnet. Since we took one bit from the host part, we are left with seven bits for the hosts. Is it enough for 50 hosts? The formula to calculate the number of hosts is 2 y — 2, with y representing the number of host bits.

Since 2 7 — 2 is , we have more than enough bits for our hosts. The range of IP addresses in this subnet is Skip to content. Before we start subnetting, we have to ask ourselves these two questions: 1. The other seven values are not. But notice the values in succession: Each value has an additional binary 1 and one less binary 0.

Each mask value, in succession, shows a mask value that reduces the number of host bits by 1 and adds 1 to the size of the subnet field. If you simply memorize each decimal value and its binary equivalent, converting masks from decimal to binary will be a breeze. In fact, you could sit down to take the exam, and before starting, go ahead and write down the information in the table so you could easily refer to it during the exam.

Given an address and mask, how many subnets are there? And how many hosts are there in a single subnet? Well, two simple formulas provide the answers, and the formulas are based on the information that you just learned how to derive:. The formulas calculate the number of things that can be numbered using a binary number and then subtract 2 for two special cases.

IP addressing conventions define that two subnets per network should not be used and that two hosts per subnet should not be used.

One reserved subnet, the subnet that has all binary 0s in the subnet field, is called the zero subnet. The subnet with all binary 1s in the subnet field is called the broadcast subnet —and it also is reserved.

Well, in fact, you can use both these subnets on a Cisco router, but it is recommended that you avoid using them. On the exam, the "right" answer is that you do not use them—hence the "minus 2" part of the 2number-of-subnet-bits - 2 formula.

In fact, the courses upon which CCNA is based now use the term discouraged instead of reserved, meaning that although those two subnets can be used, you should avoid it.

IP addressing conventions also reserve two IP addresses per subnet: the first all binary 0s in the host field and last all binary 1s in the host field addresses. No tricks exist to make these two addresses usable—they are indeed always reserved. The details of the algorithm used to answer subnetting questions about the number of hosts and subnets are summarized in the following list:. Step 3 Identify the size of the host part of the address, based on the number of binary 0s in the mask.

If the mask is tricky, use the chart of typical mask values to convert the mask to binary more quickly. Cisco Certified Expert. Mark Falco How to find how many subnets and hosts per subnet?