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What Is VLSM and Why Is It Used?

What Is VLSM and Why Is It Used?. Classful and Classless Routing. Classful (basic subnetting) Classful routing protocols require that a single network use the same subnet mask RIP v1 required that all connected networks have the same subnet mask So couldn’t subnet a subnet.

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What Is VLSM and Why Is It Used?

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  1. What Is VLSM and Why Is It Used?

  2. Classful and Classless Routing Classful (basic subnetting) • Classful routing protocols require that a single network use the same subnet mask • RIP v1 required that all connected networks have the same subnet mask • So couldn’t subnet a subnet. Classless (uses VLSM) • VLSM allows a single autonomous system to have networks with different subnet masks. • Often referred to as "subnetting a subnet"

  3. Support for Classless Routing Open Shortest Path First (OSPF) • Integrated Intermediate System to Intermediate System (Integrated IS-IS) • Enhanced Interior Gateway Routing Protocol (EIGRP) • RIP v2 • static routing • (but not RIP v1, IGRP, EGP)

  4. A Waste of Space

  5. Subnet Zero • Not recommended by CISCO with normal subnetting • Due confusion when a network and a subnet have the same addresses • OK to use subnet zero with VLSM • Cisco IOS v12.0 routers use subnet zero by default • Or use the “no ip subnet-zero” command • Not recommended by CISCO to use last subnet • Due confusion b/w a network broadcast and a subnet directed broadcast

  6. When to Use VLSM?

  7. Wasting Host Addresses on WAN Connections • Consider class C address 192.168.187.0/27 network • 3 bits for subnet = 8 – 1 = 7 useable subnets • 5 bits for hosts = 30 hosts per subnet • But we don’t want to waste 3 x 30 host subnets on the WAN connections so we subnet one of the subnets • Must therefore have VLSM enabled routers

  8. Using VLSM for WAN Connections

  9. Subnetting Details Subnet 6 (/27) further subnetted with mask of /30 to provide 8 subnets in total with 4 hosts per subnet (2 bits).

  10. Further Example • Could use 255.255.255.0 subnet mask - 172.16.0.0/24 • 255 networks with 255 hosts per network • But we would waste 253 hosts for WAN • Similar problem to version 2 TCS using RIP V1 • So use /24 subnet mask and further subnet one subnet with a /30 mask for the WAN

  11. Further Example • 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.

  12. VLSM Solution • Given 172.16.32.0/20 • Gives 16 subnets with 4096 hosts per subnet Wasteful • Further subnet one of the sixteen subnets into a /26 • giving 64 subnets with 64 hosts

  13. Subnet Details

  14. A Working VLSM Example

  15. Classless InterDomain Routing (CIDR) and VLSM • Can only subnet a subnet that is empty • Classless InterDomain Routing (CIDR) and VLSM not only prevents address waste, but also promotes route aggregation (OR summarization). • Classless routing protocols carry a prefix that consists of 32-bit IP address and bit mask in the routing updates

  16. Configuring VLSM

  17. RIP History • RIPv1 has the following limitations: • It does not send subnet mask information in its updates. • It sends updates as broadcasts on 255.255.255.255. • It does not support authentication. • It is not able to support VLSM or classless interdomain routing (CIDR).

  18. RIPv2 Features

  19. Comparing RIPv1 and RIPv2

  20. Configuring RIPv2

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