In Chapter 11, you were introduced to two advanced routing protocols: OSPF and EIGRP. Both of these protocols are classless protocols that support advanced IP addressing concepts, including variable-length subnet masking (VLSM) and route summarization. This chapter focuses on these two advanced concepts. | Variable-Length Subnet Masking CCNA CISCO CERTIFIED NETWORK ASSOCIATE 12 Advanced IP Addressing Two-Minute Drill Self Test 2 Chapter 12 Advanced IP Addressing In Chapter 11 you were introduced to two advanced routing protocols OSPF and EIGRP. Both of these protocols are classless protocols that support advanced IP addressing concepts including variable-length subnet masking VLSM and route summarization. This chapter focuses on these two advanced concepts. CERTIFICATION OBJECTIVE VLSM VLSM defined in RFC 1812 allows you to apply different subnet masks to the same class address. For instance a good mask for point-to-point links is which provides for two host addresses in each network. A good mask for a LAN connection might be which provides for 62 host addresses for each network. Using a mask for a LAN connection will not give you enough host address and using a mask on a point-to-point connection wastes addresses. One solution would be to divide the mask values in the middle to limit the waste of addresses but this doesn t scale well. VLSM solves this problem by enabling you to use different subnet mask values on the same class address space. The following sections cover the advantages that VLSM provides as well as how to use VLSM in your own network. Features of VLSM VLSM enables you to have more than one mask for a given class of address albeit a class A B or C network number. Classful protocols such as RIPv1 and IGRP do not VLSM allows you to use more than one subnet mask for a given class address. Remember the two major advantages of VLSM. support VLSM. To deploy VLSM requires a routing protocol that is classless BGP EIGRP IS-IS OSPF or RIPv2 for instance. VLSM provides two major advantages More efficient use of addressing Ability to perform route summarization As these bullets suggest one advantage of VLSM is that it allows you to make more efficient use of your IP addressing. Figure .
