111 What Is VLSM and Why Is It Used?

222 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"

333 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)

444 A Waste of Space

555 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

666 When to Use VLSM?

777 Wasting Host Addresses on WAN Connections Consider class C address /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

888 Using VLSM for WAN Connections

999 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 subnet mask / 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 /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 /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 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