1 VLSM ( Virtual Length Subnet Mask ) VLSM dapat digunakan routing protocol seperti OSPF, Integrated IS-IS, EIGRP, RIP v2,dan routing static. Beikut ini ilustrasi digunakan : No ID Range 0 192.168.187.0 192.168.187.1-192.168.187.30 1 192.168.187.32 192.168.187.33-192.168.187.62 2 192.168.187.64 192.168.187.65-192.168.187.94 3 192.168.187.96 192.168.187.97-192.168.187.126 4 192.168.187.128 192.168.187.129-192.168.187.158 5 192.168.187.160 192.168.187.161-192.168.187.190 6 192.168.187.192 192.168.187.193-192.168.187.222 7 192.168.187.224 192.168.187.225-192.168.187.254 Terdapat subnetting dengan network id 192.168.187.0/27 seperti pada tabel diatas yang sudah terdapat pengelompokannya. Pada gambar diatas, untuk menghubungkan antara router ( point to point ) diperlukan 2 IP, jika digunakan pada network diatas dengan subnet /27, host yang dimiliki 30 IP, sehingga terjadi pemborosan IP. Untuk mengatasinya, kita dapat memilih salah satu network misalkan no 6 : 192.168.187.192/27 akan di sub subnet kembali.
2 Hasilnya : Lalu bagaimana cara menghitungnya???? Solusi : Network 192.168.187.192 /27 akan disubnet kembali menjadi 192.168.187.192/30 hanya memiliki 2 host dari 30 host. Jika dirubah ke binary : 3 oktet pertama dari network tidak berubah, jadi dibiarkan dalam bentuk desimal 192.168.187.11 000000 255.255.255.11 100000 = /27 Kemudian host diperkecil menjadi 2 dimana subnet /30 Sehingga 192.168.187.11 000000 255.255.255.11 111100 =/30 Dan terjadi network baru dimana range dimulai dari: 192.168.187.11 000000 255.255.255.11 111100 =/30 Dan network 192.168.187.192/30 Range nya adalah 1. 192.168.187.11 000000-192..168.187.11 011100 atau jika didesimalkan menjadi 192.168.187.192 192.168.187.220
3 2. Dan karena subnet nya /30, ingat bit 0 sisanya hanya 2, sehingga host menjadi kelipata 2 2 = 4 3. Subentting selesai hasilnya adalah : Mudah bukan. Gambar router akan menjadi : Contoh lain : Terdapat router seperti pada gambar dibawah ini :
4 Gunakan VLSM untuk efisiensi IP address dan mengurangi informasi router. Jika dilihat dari tabel diatas dapat dianalisa: Router KL membutuhkan 30 host, router perth 60 host dan router sydney serta singapura membutuhkan 12 host. Dalam penggunaan VLSM yang menjadi tolak ukuran adalah host yang paling besar. Maka host yang paling besar adalah Router pert dengan 60 host. 60 host yang mendekati adalah 2 6 = 64 host, network yang digunakan adalah kelas C, misalkan digunakan netowork 192.168.11.0. Maka subnetting untuk 64 host adalah 255.255.255.192, ingat dibutuhkan hanya 6 bit 0 pada subnet mask sehingga menjadi 255.255.255.11000000 No ID Range Broadcast 0 192.168.11.0 192.168.11.1-192.168.11.62 192.168.11.63 1 192.168.11.64 192.168.11.65-192.168.11.126 192.168.11.127 2 192.168.11.128 192.168.11.129-192.168.11.190 192.168.11.191 3 192.168.11.192 192.168.11.193-192.168.11.254 192.168.11.255 Dari hasil diatas untuk router pert telah tersedia, dapat dipilih sesuai kehendak, diasumsikan untuk router perth menggunakan network 192.168.11.0 Sisanya adalah no 1,2,3 tahap selanjutnya melihat network yang terbesar, yaiut pada router KL sebanyak 28 host. 28 host yang mendekati adalah 2 5 = 32. Untuk KL kita gunakan network 192.168.11.64 Pada awalmya diubah terlebih dahulu menjadi binary IP = 192.168.11.11 000000 SM = 255.255.255.11 000000 = 255.255.255.192
5 Disubnet kembali menjadi 28 host IP = 192.168.11. 11 000000 SM = 255.255.255.11 100000 =255.255.255.224 Rangenya adalah = 192.168.11.010 00000 192.168.11.010 11111 = 192.168.11.64-192.168.11.95 dan 192.168.11.011 00000 192.168.11.011 11111 = 192.168.11.96 192.168.11.127 Ingat kelipatannya adalah 32 dari 2 5, sehingga dari hasil diatas terdapat dua range yang dipilih, anda dapat memilih yang mana saja. Misalkan saya pilih Netowrk 192.168.11.64 /27 sebagai network untuk router KL Yang tersisa kini network 192.168.11.96/27 untuk router sydney dan singapore yang masing masing jumlah host adalah 12, 12 yang mendekati adalah 2 4 Diuraikan kembali menjadi binary : IP = 192.168.11.011 00000 =192.168.11.96 SM = 255.255.255.111 00000 Yang dibutuhkan kini hanya 16 host maka IP = 192.168.11. 011 000000 SM = 255.255.255. 111 10000 Sehingga rangenya adalah 192.168.11.011 0 0000 011 0 1111 = 192.168.11.96 192.168.11.111 192.168.11.011 1 0000 011 1 1111 =192.168.11.112 192.168.11.128 Dengan subnet mask 255.255.255.240 =/26 Dapat diperoleh Untuk router sydney menggunakan network = 192.168.11.96/28
6 Dan router singapora menggunakan netowrk = 192.168.11.112/28 Hasil Router Perth = 192.168.11.0 /26 Router KL = 192.168.11.64/27 Latihan: Soal pertama Gunakan VLSM untuk efisiensi IP address dan mengurangi informasi router. Soal kedua Given the same network and requirements as the example above, develop a subnetting scheme using VLSM, given: neta: must support 14 hostsnetb: must support 28 hostsnetc: must support 2 hostsnetd: must support 7 hostsnete: must support 28 host Soal Ketiga
7 QUESTION NO: 35 On the topic of VLSM, which one of the following statements best describes the concept of the route aggregation? A. Deleting unusable addresses through the creation of many subnets. B. Combining routes to multiple networks into one supernet. C. Reclaiming unused space by means of changing the subnet size. D. Calculating the available host addresses in the AS. Answer: B In the networking world route aggregate means combining routes to multiple networks into one. This is also known as route summarization or supernetting. It is normally used to reduce the number of route entries in the routing table by advertising numerous routes into one larger route. Reference: CCNA Self-Study CCNA ICND exam certification Guide (Cisco Press, ISBN 1-58720-083-X) Page 236. QUESTION NO: 36 You have a single Class C IP address and a point-to-point serial link that you want to implement VLSM on. Which subnet mask is the most efficient? A. 255.255.255.0 B. 255.255.255.240 C. 255.255.255.248 D. 255.255.255.252 E. 255.255.255.254 Answer: D For a single point to point link, only 2 IP addresses are required, one for the serial
8 interface of the router at each end. Therefore, the 255.255.255.252 subnet mask is often used for these types of links, as no IP addresses are wasted. You have a network that supports VLSM and you need to reduce IP address waste in your point to point WAN links. Which of the masks below would you use? A. /38 B. /30 C. /27 D. /23 E. /18 F. /32 Answer: B For a single point to point link, only 2 IP addresses are required, one for the serial interface of the router at each end. Therefore, the 255.255.255.252 subnet mask is often used for these types of links because no IP addresses are wasted. The subnet mask 255.255.255.252 is a /30, so answer B is correct. A. The largest mask that can be used is the single IP host mask, which is /32. It is not possible to use a /38 mask, unless of course IPv6 is being used. C, D, E. These masks will provide for a larger number of host addresses, and since only 2 IP addresses are needed for a point to point link, these extra addresses are wasted. F: No available host addresses with a /32 mask You are a network administrator and you need to implement a routing protocol on your network that provides: Scalability VLSM support Minimal overhead Support for connecting networks using routers of multiple vendors Which of the following routing protocol would best serve your needs? A. VTP B. RIP version 1 C. EIGRP D. OSPF E. IGRP F. CDP Answer: D Since one of the requirements is that the routing protocol must support other vendors, our only choices are RIP and OSPF. Since RIP version 1 does not support VLSM, OSPF is the only choice. A. VTP is the VLAN Trunking Protocol. This is not a routing protocol. B. RIP version one does not support VLSM. Note that RIPv2 does support VLSM, and
9 would be a valid choice. C, E. EIGRP and IGRP are Cisco proprietary routing protocols, and are not supported by other vendors. F. CDP is the Cisco Discovery Protocol, which is used to exchange information between Cisco devices. It can only be used between Cisco routers and switches, and it is not a routing protocol. Which of the routing protocols shown below support both VLSM and route summarization? (Select three) A. IGRP B. EIGRP C. RIP v1 D. RIP v2 E. OSPF F. VTP G. CDP Answer: B, D, E EIGRP and OSPF support Variable Length Subnet Masks (VLSM) and provide for both automatic and manual route summarization configurations. RIPv2 is an enhanced version of RIP, and overcame some of the limitations of RIP by introducing support for VLSM. A, C. IGRP and RIP are relatively old and simplistic routing protocols that were developed before the concepts of VLSM and route summarization. F. VTP is the VLAN Trunking Protocol, used in switched LAN environments to carry VLAN information. It is not a routing protocol. G. CDP is the Cisco Discovery Protocol, used between neighboring Cisco devices to automatically discover information. It is not a routing protocol. Which of the following routing protocols support the use of VLSM (Variable Length Subnet Masking)? (Select three) A. RIPv1 B. EIGRP C. OSPF D. IGRP E. RIPv2 Answer: B, C, E Which of the following routing protocols do NOT support VLSM (variable length subnet masking)? (Choose all that apply). A. RIPv1 B. IGRP C. EIGRP D. OSPF
10 E. IS-IS F. RIPv2 You need to implement the use of a routing protocol that meets the following requirements: 2. Converges quickly 3. Supports VLSM, CIDR, IP, and IPX. 4. Uses minimal bandwidth for routing updates. Which one of the following routing protocols would be the best choice? A. RIPv1 B. RIPv2 C. IGRP D. OSPF E. EIGRP Answer: E EIGRP would be the best choice as it provides support for VLSM and CIDR, has faster convergence times than other protocols, is scalable, and supports IP, IPX, and Appletalk. EIGRP is a Cisco proprietary routing protocol, so it will not work with other vendors. However, the requirements of the question made no mention of the use of non-cisco routers, so it will not be an issue in this case. A, C. Both of these routing protocols do not support VLSM. B. While RIPv2 supports VLSM, it provides no support for IPX. The IPX RIP protocol is similar in function to RIP version 1. Both versions of RIP are also consume more bandwidth than EIGRP. D. OSPF does not support IPX. Testking has four offices, each with its own network, as shown in the graphic. Three of the networks have approximately 50 hosts each, and one network has 10 hosts. The multi-vendor routers are connected by serial links that use separate
11 subnetwork numbers. The Testking network has leased one Class C address to be used for all networks and serial links, and they do not wish to replace any of their existing routers. Which routing protocol would be most appropriate for this scenario? A. TCP/IP B. RIP version 1 C. RIP version 2 D. IGRP E. EIGRP F. All of the above are acceptable Answer: C The question describes 2 important requirements. The first is the fact that a routing protocol that supports VLSM is needed, as specified by the fact that one class C address range is to be used for all networks. The second important requirement is that routers from multiple vendors are being used, so the routing protocol chosen must be nonproprietary. RIP version 2 is a standards based routing protocol that supports variable length subnet masking (VLSM). A. This is not a routing protocol. B. RIP version 1 does not support VLSM D, E. Although these both support VLSM, IGRP and EIGRP are Cisco proprietary routing protocols which are not supported by other router vendors. Assume that RIP v1 is the only routing protocol in use. What is the Maximum number of usable IP address that can be supported on each LAN if the Testking network is using one Class C address block? A. 14
12 B. 16 C. 30 D. 32 E. 62 F. 64 Answer: A RIP version 1 does not support VLSM information, so all networks must have the same subnet mask. In the network above, there are a total of 12 networks (6 LANs and 6 different point to point WAN connections). Therefore, if each of the 12 networks use the 255.255.255.240 subnet mask, there will be a total of 16 networks with 14 usable hosts on each LAN. B. There are only 14 usable IP addresses in the 255.255.255.240 subnet mask, not 16, since we must subtract 2 for the network and broadcast IP addresses. C, E. These options will not provide enough separate networks. A total of 12 are required due to the use of a protocol that does not support VLSM. D, F. These options omit the fact that we must subtract 2 addresses from the usable range for the network and broadcast IP addresses for each subnet. Which dynamic routing protocol should be recommended for the Testking network shown in the graphic above? (Choose three) A. OSPF B. RIP version 1 C. RIP version 2 D. IGRP E. EIGRP Answer: A, C, E In this network, the 192.168.23.0/24 network is subnetted into two other networks. Because this class C network is being subnetted, a routing protocol that supports variable length subnet mask information is required. OSPF, EIGRP, and RIP version 2 all support VLSM information to be shared across the network. B, D. RIP version 1 and IGRP do not support VLSM, which will be required in order for
13 this network to have the two LANs both be reachable.