VLSM

• Variable-length subnet masking (VLSM) – using more than one mask in a single classful network.
• An example would be an internetwork that uses the following ip scheme:
  • 10.1.4.0/30 on serial interfaces
  • 10.1.5.0/30 on serial interfaces
  • 10.1.6.0/24 on LAN interfaces
  • 10.1.7.0/24 on LAN interfaces
• Some routing protocols that support VLSM (which also implies that they send the mask in the routing updates):
  • RIP-2
  • EIGRP
  • OSPF

Overlapping VLSM Subnet

• With VLSM, there is potential that there are overlapping subnets created due to careless planning.
• In order to verify correct ip addressing and no overlapping subnets, use the following methods:
  1. Find the range of the IP address for each subnet by calculating the subnet number and subnet broadcast
  2. Compare the range of addresses and make sure there is no overlap.

Designing a Subnetting Scheme Using VLSM

Example:

Create a IP addressing scheme for the following network, given the Class C network of 204.15.5.0/24, satisfying the requirements for number of hosts. Use VLSM to develop the subnetting scheme.

NetA: must support 14 hosts
NetB: must support 28 hosts
NetC: must support 2 hosts
NetD: must support 7 hosts
NetE: must support 28 host

First determine the mask needed to satisfy the required number of hosts for each network.

NetA: requires a /28 (255.255.255.240) mask to support 14 hosts (24 Network bits, 4 subnet bits, 4 host bits gives you at least 14 hosts)
NetB: requires a /27 (255.255.255.224) mask to support 28 hosts (24 N-bits, 3 S-bits, and 5 host bits gives you at least 24 hosts)
NetC: requires a /30 (255.255.255.252) mask to support 2 hosts
NetD: requires a /28 (255.255.255.240) mask to support 7 hosts
NetE: requires a /27 (255.255.255.224) mask to support 28 hosts

Start assigning subnet numbers with the largest subnet first (the one with the smallest number of subnet bits)

NetB: 204.15.5.0/27 host address range 1 to 30
NetE: 204.15.5.32/27 host address range 33 to 62
NetA: 204.15.5.64/28 host address range 65 to 78
NetD: 204.15.5.80/28 host address range 81 to 94
NetC: 204.15.5.96/30 host address range 97 to 98

Adding a New Subnet to an Existing Design

VLSM Configuration

• VLSM cannot be enabled or disabled on a router. Instead, it is used by configuring at least two ip addresses on interfaces with addresses in the same classful network but with different subnet mask.
• Support for VLSM by routing protocols is not enabled or disabled either. It is simply a property inherent with the routing protocol

Manual Route Summarization

• Reduces the size of the routing table by aggregating a group of routes into a single route that is advertised to the other routers. As a result, there is speedier process in routing packets as well as less amount of processing power is consumed.
• Needs to be configured in order for it to occur

• To manually configure summary route with EIGRP (or RIP v2), use the interface subcommand ip summary-address

R1(config-if)#ip summary-address eigrp 1 10.2.0.0 255.255.0.0

R2(config-if)#ip summary-address eigrp 1 10.3.0.0 255.255.0.0

Route Summarization Strategies

The following example illustrates a method of summarize a group of subnets.

10.3.4.0/24
10.3.5.0/24
10.3.6.0/24
10.3.7.0/24

1. Convert the addresses to binary
2. Find the all the common bits from left to right
3. Convert all the bits that are in common back to decimal, and leave all the rest of the bits to zero.
4. The number of bits that are in common will be their subnet mask

10.3.4.0: 00001010 00000011 00000100 00000000
10.3.5.0: 00001010 00000011 00000101 00000000
10.3.6.0: 00001010 00000011 00000110 00000000
10.3.7.0: 00001010 00000011 00000111 00000000

The bits in red are the common bits. When the bits in red are converted back to decimal, the end result is 10.3.4.0. This is the summary subnet.

To calculate the mask, count all the bits in red. There are 22 bits. Therefore the summary route is 10.3.4.0/22 or subnet mask 255.255.252.0

Autosummarization

• Autosummarization by definition only allows summarization such that the summary route is an entire classful class A, B, or C network.
  • Versus Route (manual) summarization, which can only be supported by classless routing protocols. It requires the subnet mask information to be advertised to the routing protocols update.
• Requires networks to be contiguous in order for autosummarization to work properly.
• It occurs automatically without any specific configuration
• RIP v1 cannot perform manual route summarization because it is a classful routing protocol.

Discontiguous Network

• A discontiguous network is a classful network where packets sent to a a pair of subnets has to pass through a different classful network. For example, a packet sent from 10.2.0.0 must pass through a network in a 172.16.0.0 network to get to network 10.3.0.0.
  • 10.2.0.0 —— 172.16.0.0 —— 10.3.0.0
  • Autosummarization causes a problem on a discontiguous network.

Classful and Classless Defaults

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