![]() ![]() Now look to the upper right of the figure, to subnet 172.16.3.0/24. As you can see just by looking at the subnet IDs to the right, all the subnets referenced with the arrowed lines are within that same range of addresses. That subnet includes addresses from the subnet ID of 172.16.4.0 through the subnet broadcast address of 172.16.7.255. Just to complete the example, first look at subnet 172.16.4.0 on the lower left. Not only do these more advanced classless routing protocols support VLSM, but they also support manual route summarization, which allows a routing protocol to advertise one route for a larger subnet instead of multiple routes for smaller subnets. The classless routing protocols, as noted in Table 22-2, are the newer, more advanced routing protocols. To effectively support VLSM, the routing protocol needs to advertise the correct mask along with each subnet so that the receiving router knows the exact subnet that is being advertised.īy definition, classless routing protocols advertise the mask with each advertised route, and classful routing protocols do not. Without mask information, the router receiving the update would be confused.įor example, if a router learned a route for 10.1.8.0, but with no mask information, what does that mean? Is that subnet 10.1.8.0/24? 10.1.8.0/23? 10.1.8.0/30? The dotted-decimal number 10.1.8.0 happens to be a valid subnet number with a variety of masks, and because multiple masks can be used with VLSM, the router has no good way to make an educated guess. To support VLSM, the routing protocol must advertise the mask along with each subnet. ![]() With private networks, as defined in RFC 1918, running out of addresses is not as big a negative, because you can always grab another private network from RFC 1918 if you run out.īefore you can deploy a VLSM design, you must first use a routing protocol that supports VLSM. With public networks, the address savings help engineers avoid having to obtain another registered IP network number from regional IP address assignment authorities. ![]() VLSM can be helpful for both public and private IP addresses, but the benefits are more dramatic with public networks. ![]() By wasting fewer addresses, more space remains to allocate more subnets. This flexibility reduces the number of wasted IP addresses in each subnet. For example, for subnets that need fewer addresses, the engineer uses a mask with fewer host bits, so the subnet has fewer host IP addresses. Because a mask defines the size of the subnet (the number of host addresses in the subnet), VLSM allows engineers to better match the need for addresses with the size of the subnet. VLSM provides many benefits for real networks, mainly related to how you allocate and use your IP address space. In that case, the design does not use VLSM. However, Class A network 10.0.0.0 uses only one mask, and Class A network 11.0.0.0 uses only one mask. Oddly enough, a common mistake occurs when people think that VLSM means “using more than one mask in some internetwork” rather than “using more than one mask in a single classful network.” For example, if in one internetwork diagram, all subnets of network 10.0.0.0 use a 255.255.240.0 mask, and all subnets of network 11.0.0.0 use a 255.255.255.0 mask, the design uses two different masks. All subnets are of Class A network 10.0.0.0, with two masks being used, therefore meeting the definition of VLSM. Figure 22-1 shows a typical choice of using a /30 prefix (mask 255.255.255.252) on point-to-point serial links, with mask /24 (255.255.255.0) on the LAN subnets. ![]()
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