Then you take the defined network subnet mask. Convert that to binary as well:. CIDR notation is determined by counting the 1s within the subnet mask. As you can see in the string above, there are 24 1s. Once the subnet mask is converted, you know that every binary value within an IP address that matches a 1 in the subnet mask is part of the network address.
Any binary value within an IP address that matches a 0 in the subnet mask is part of the host address. In this example, that means any value in the last octet is a host. You basically do a logical AND operation. That means that if both values are 1, the result is a 1. If both values are 0, the result is a 0. And if the values differ 1 and 0 , the result is a 0.
So, for our example, we get the following:. Translating this back into decimal gives us This would be written as Deborah Littlejohn Shinder, Variable length subnetting is the process of subdividing subnets. The process is the same as when subnetting classful network addresses, as we have illustrated using a Class A, B, and C network.
Variable length subnetting or nonclassful subnetting allows us to create subnets of unequal size to avoid wasting IP addresses within the network range. By creating subnets with a varying number of available host addresses, you can better utilize your allocated addressing space: This recursive method of subdividing uses the subnetting principle of taking bits from the host address space to add to the network address space.
In Chapter 1 , we emphasized learning the fundamentals of subnetting , starting with converting decimal to binary and binary to weighted binary. You also learned how to work with dotted decimal notation. You can probably see now why this was so important. Not only is it important for basic networking, it is required for variable length subnetting. With the growing shortage of IP addresses worldwide, new schemes have been devised to help make better use of networks.
We can no longer afford simply to leave thousands of IP addresses unused, so creating subnets from subnets makes efficient use of these IP addresses. The process of creating variable length subnets is not complex, but it requires full attention to detail. A few tips can make this process easier, both in real world scenarios and on the exam. What is your current subnet mask? How many subnets do you currently have and how many host addresses are available on each subnet? Next determine your new configuration needs.
Remember, this is not a task to be taken lightly, as it often involves changing cabling, routers, and other network configurations. If at all possible, work with your current network configuration as your starting point. Finally, determine your subnetting scheme. These requirements help you define your needs and develop solutions to match those needs. This planning and attention to detail will pay off—by reducing errors and networking configuration problems in a real network and by helping you discern the correct answers to exam questions.
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Variable length subnetting is not difficult when you understand the foundations of subnetting, but it still requires strict attention to detail. IP address subnetting applies to any IP address. Subnetting also allowed a single Class C IP address to be used on small LANs having fewer than hosts connected by routers instead of bridges. Bridges would simply shuttle frames among all of the ports on the bridge, but routers, as packet layer devices, determine the output interface for a packet based on the network portion of the IP address.
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If only one address is assigned to the entire site, but two LANs on the site are connected through a router, then the address must be subnetted so that the router functions properly. Basically, you need to create two distinct address spaces, and the IP host addresses assigned on each LAN segment must be correct as well.
Subnetting is done using an IP address mask. The mask is a string of bits as long as the IP address 32 bits in the case of IPv4. A mask of This is no longer true, and the effect is to restrict masks to the ending values listed. Note that Once the 1 bits stop, the rest of the subnet mask must be set to all 0 bits. Subnet masks can be written in as many forms as there are for IP addresses: dotted decimal notation, bit string, octal, or hexadecimal. In all cases it is possible to change the default mask to move the boundary between the network and host portions of the IP address to wherever the device needs to see it.
All devices, whether hosts or routers, which need to route the packets within the subnetted network, must have identical masks.
All routing protocols in wide use today exchange subnet mask information together with routing information. The use of the default masks for the original classful IP address space is shown in Table 5. The more bits, the more network identifiers, and the fewer bits, the fewer host identifiers possible. Table 5. Subnetting moves the boundary between the network and host for a particular classful IP address to the right of the position where the boundary is normally found.
We will see later that supernetting moves the boundary between network and host for a particular classful IP address to the left of this position. It is important to realize that subnetting does not change anything with respect to the outside world. Internet routers still deliver the packets as before. It is the customer or site router that applies the subnet mask and delivers packets to the subnets. Instead of the usual two parts of the IP address, network, and host, we now have network, subnet, and host.
Look at a simple LAN The subnet creates two equal-sized subnets, but the Internet routers deliver packets as before. If this bit is 0, the first subnet is intended, and if the bit is 1, then the second subnet is intended. The hosts must be numbered according to the subnet, naturally, and all have the same subnet mask so they can determine which addresses are still on their subnet same NetID and which are not different NetID.
Figure 5. Subnetting a LAN, showing how the value of the initial bits determines the subnet. Host addresses, if assigned manually, must follow the subnet mask convention. Many implementations will not allow the assignment of the first subnet address the network or the last broadcast. A LAN with hosts subnetted into two subnets only yields host addresses per subnet, not A sometimes tricky subnet issue is determining exactly what the subnet address all 0 bits after the mask and broadcast address all 1 bits after the mask are for a given IP address and subnet mask.
This can be difficult because subnet masks do not always fall on byte boundaries as do classful addresses. An IP address like Consider the address What is the subnet and broadcast address for this subnet? What range of host addresses can be assigned to this subnet? These questions come up all the time, and there are utilities available on the Internet that do this quickly. The first thing to do is to mask out the network portion of the IP address with the subnet mask by writing down the mask bits.
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Next, it is easy to form the subnet and broadcast address for the subnet by setting the rest of the bits in the address the host bits first to all 0 bits network and then to all 1 bits broadcast. The resulting address range forms the limits of the subnet. Finding subnet host address range, showing those available for host assignment.
Many routers allow the use of subnet and broadcast addresses as if they were host addresses. These answers are important when subnetting the IP address space because care is needed to assign host addresses to the proper subnets and router interfaces. If subnetting is a problem, the bit block class C networks , or the bit block class B networks of private address space can be used. Since most IP software modules in use today were developed after that time, they do understand how to do subnetting.
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So go ahead and use the 10 network for private addressing unless you have good reasons to do otherwise. By using the bit block, you have 24 bits to play with when designing a private addressing scheme.