router. Therefore there would be 252 addresses wasted. If VLSM were used in this example, a 24-bit mask would still work on the LAN segments for the 250 hosts. A 30-bit mask could be used for the WAN link because only two host addresses are needed. In Figure the subnet addresses used are those generated from subdividing the 172.16.32.0/20 subnet into multiple /26 subnets. The figure illustrates where the subnet addresses can be applied, depending on the number of host requirements. For example, the WAN links use subnet addresses with a prefix of /30. This prefix allows for only two hosts, just enough hosts for a point-to-point connection between a pair of routers. To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26 subnets. In this example, 172.16.33.0/26 is further subnetted with a prefix of /30. This provides four more subnet bits and therefore 16 (24) subnets for the WANs. Figure illustrates how to work through a VLSM masking system. VLSM allows the subnetting of an already subnetted address. For example, consider the subnet address 172.16.32.0/20 and a network needing ten host addresses. With this subnet address, there are over 4000 (212 – 2 = 4094) host addresses, most of which will be wasted. With VLSM it is possible to further subnet the address 172.16.32.0/20 to give more network addresses and fewer hosts per network. For example, by subnetting 172.16.32.0/20 to 172.16.32.0/26, there is a gain of 64 (26) subnets, each of which could support 62 (26 – 2) hosts. Use this procedure to further subnet 172.16.32.0/20 to 172.16.32.0/26: Step 1 Write 172.16.32.0 in binary form. Step 2 Draw a vertical line between the 20th and 21st bits, as shown in Figure . /20 was the original subnet boundary. Step 3 Draw a vertical line between the 26th and 27th bits, as shown in Figure . The original /20 subnet boundary is extended six bits to the right, becoming /26. Step 4 Calculate the 64 subnet addresses using the bits between the two vertical lines, from lowest to highest in value. The figure shows the first five subnets available. It is important to remember that only unused subnets can be further subnetted. If any address from a subnet is used, that subnet cannot be further subnetted. In the example, four subnet numbers are used on the LANs. Another unused subnet, 172.16.33.0/26, is further subnetted for use on the WANs. Lab Activity Lab Exercise: Calculating VLSM Subnets In this lab, students will use variable-length subnet mask (VLSM) to support more efficient use of the assigned IP addresses and to reduce the amount of routing information at the top level.
Content 1.1 VLSM 1.1.5 Route aggregation with VLSM When using VLSM, try to keep the subnetwork numbers grouped together in the network to allow for aggregation. This means keeping networks like 172.16.14.0 and 172.16.15.0 near one another so that the routers need only carry a route for 172.16.14.0/23. The use of Classless InterDomain Routing (CIDR) and VLSM not only prevents address waste, but also promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997. Figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points, using a prefix address, until the entire network is advertised as a single aggregate route, 200.199.48.0/22. Route summarization, or supernetting, is only possible if the routers of a network run a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry a prefix that consists of 32-bit IP address and bit mask in the routing updates. In Figure , the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization, 200.199.48.0/22 or 11001000.11000111.0011. For summarization to work properly, carefully assign addresses in a hierarchical fashion so that summarized addresses will share the same high-order bits. Remember the following rules:
  1. A router must know in detail the subnet numbers attached to it.
  2. A router does not need to tell other routers about each individual subnet if the router can send one aggregate route for a set of routers.
  3. A router using aggregate routes would have fewer entries in its routing table.
VLSM allows for the summarization of routes and increases flexibly by basing the summarization entirely on the higher-order bits shared on the left, even if the networks are not contiguous. The graphic shows that the addresses, or routes, share each bit up to and including the 20th bit. These bits are colored red. The 21st bit is not the same for all the routes. Therefore the prefix for the summary route will be 20 bits long. This is used to calculate the network number of the summary route. Figure shows that the addresses, or routes, share each bit up to and including the 21st bit. These bits are colored red. The 22nd bit is not the same for all the routes. Therefore the prefix for the summary route will be 21 bits long. This is used to calculate the network number of the summary route. Web Links Internetworking Design Basics http://www.cisco.com/en/US/products/sw/
iosswrel/ ps1828/products_configuration_
guide_ chapter09186a00800ca569.html Configuring IP Routing Protocol-Independent Features http://www.cisco.com/en/US/products/sw/
iosswrel/ ps1835/products_configuration_
guide_chapter09186a00800ca765.html
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Content 1.1 VLSM 1.1.6 Configuring VLSM If VLSM is the scheme chosen, it must then be calculated and configured correctly. In this example allow for the following: Network address: 192.168.10.0 The Perth router has to support 60 hosts. In this case, a minimum of six bits are needed in the host portion of the address. Six bits will yield 62 possible host addresses, 26 = 64 – 2 = 62, so the division was 192.168.10.0/26. The Sydney and Singapore routers have to support 12 hosts each. In these cases, a minimum of four bits are needed in the host portion of the address. Four bits will yield 14 possible host addresses, 24 = 16 – 2 = 14, so the division is 192.168.10.96/28 for Sydney and 192.168.10.112/28 for Singapore. The Kuala Lumpur router requires 28 hosts. In this case, a minimum of five bits are needed in the host portion of the address. Five bits will yield 30 possible host addresses, 25 = 32 – 2 = 30, so the division here is 192.168.10.64/27. The following are the point-to-point connections: There is sufficient host address space for two host