In IPv6, the network prefix performs a similar function as the subnet mask in IPv4, with the prefix length representing the number of bits in the address. Since the introduction of CIDRs, however, assigning an IP address to a network interface requires both an address and its network mask. Financial Fitness and Health Math Other. When the mask is tricky, you have two alternatives for deciding how many host bits are defined:.
These are the only nine valid decimal values used in a subnet mask. Converting a mask to binary without having to convert from decimal to binary will be much faster. Table lists the only valid decimal values in a mask and their binary equivalents. Memorizing these values will help you convert masks from between their decimal and binary forms more quickly on the exam.
Without the use of a calculator, PC, or decimal-to- binary conversion chart , binary conversion of a subnet mask becomes easy after memorizing this chart. The binary equivalents of and decimal 0 are obvious. 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. The -2 in the formula accounts for two addresses the subnetwork address and the broadcast address which cannot be assigned to hosts. The network To determine how many subnets we can. Our second step will be to calculate the new subnet mask, our previous subnet mask was Since we have borrowed 3 bits from the host portion our new subnet mask will be We have discussed in detail the conversion process of binary to decimal and vice versa.
When performing IP subnetting we will refer to the picture shown below which is very handy in this process. So our original subnet mask was We can quickly convert An octet which is in decimal will be in binary so We will set the first 3 bits of the last octet to 1 and last octet will now be which from the table above will be in decimal. So our new subnet mask is Our third step will be to determine the subnet multiplier which is fairly simple.
Take a look at how a Class B network might be subnetted. If you have network Extending the mask to anything beyond You can quickly see that you have the ability to create a lot more subnets than with the Class C network.
You use five bits from the original host bits for subnets. This allows you to have 32 subnets 2 5. After using the five bits for subnetting, you are left with 11 bits for host addresses. This allows each subnet so have host addresses 2 11 , of which could be assigned to devices. Note : In the past, there were limitations to the use of a subnet 0 all subnet bits are set to zero and all ones subnet all subnet bits set to one. Some devices would not allow the use of these subnets.
Cisco Systems devices allow the use of these subnets when the ip subnet zero command is configured. Now that you have an understanding of subnetting, put this knowledge to use. Your task is to determine if these devices are on the same subnet or different subnets. You can use the address and mask of each device in order to determine to which subnet each address belongs. Looking at the address bits that have a corresponding mask bit set to one, and setting all the other address bits to zero this is equivalent to performing a logical "AND" between the mask and address , shows you to which subnet this address belongs.
In this case, DeviceA belongs to subnet Given the Class C network of Looking at the network shown in Figure 3 , you can see that you are required to create five subnets. The largest subnet must support 28 host addresses. Is this possible with a Class C network? And if so, then how? You can start by looking at the subnet requirement.
In order to create the five needed subnets you would need to use three bits from the Class C host bits. Two bits would only allow you four subnets 2 2. Since you need three subnet bits, that leaves you with five bits for the host portion of the address. How many hosts does this support?
This meets the requirement. Therefore you have determined that it is possible to create this network with a Class C network. An example of how you might assign the subnetworks is:. In all of the previous examples of subnetting, notice that the same subnet mask was applied for all the subnets. This means that each subnet has the same number of available host addresses.
You can need this in some cases, but, in most cases, having the same subnet mask for all subnets ends up wasting address space. For example, in the Sample Exercise 2 section, a class C network was split into eight equal-size subnets; however, each subnet did not utilize all available host addresses, which results in wasted address space. Figure 4 illustrates this wasted address space. Figure 4 illustrates that of the subnets that are being used, NetA, NetC, and NetD have a lot of unused host address space.
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:.
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