VLSM Variable Length subnet masks

1
VLSMVariable Length subnet masks

• Subnetting the subnet

2
IP Address Design Example

• Company with 200 hosts
• Assigned class C network 195.10.1.0/24
• Want 6 different LANs
• Subnet class C network using mask 255.255.255.224
  (/27)
• Whole network has the same subnet mask or
  supports equal number of hosts on all the
  sub-networks
• Is this a problem?
• Why?
• What should be done to solve it ?

3
VLSM (Variable Length Subnet masks)

• VLSM Subnets a subnet
• VLSM divides a network into subnets of various
  sizes to prevent wasting IP addresses
• Uses up wasted addresses
• Conserves address space on WAN links
• Point-to-point link only needs 2 addresses
• With VLSM point-to-point links can use a /30 mask

4
VLSM

• Variable Length Subnet Masking allows division of
  address space based on the size of networks
• Start with network requiring the most addresses
• Create a subnet mask
• (use CIDR Classless Inter Domain Routing)
• Subnet the subnet as needed to provide address
  space required for other subnets
• Be logical start at beginning or end or address
  space
• Addresses must be contiguous to enable route
  summarization

5

• Question
• If an ISP has a 200.1.1.0 Network, he wants to
  give assign to 5 customers who need 20, 12, 6,
  25, 18 hosts.
• What should the subnet mask be?
• What is the address range in each subnet?
• What should the routing table entries be?
• Can it be done efficiently to conserve IP
  addresses?

6
VLSM - example
7
(No Transcript)
8
Practice VLSM

• You are required to connect two networks in
  Brisbane to those in Perth, having the following
  requirements
• Brisbane needs to support 25 hosts each on the
  two networks
• Perth office needs to support 5 and 10 hosts in
  its two offices
• You may use only 128.186.1.0 /24

9
VLSM
Office hosts host bits usable hosts subnet
Brisbane 25 5 30 /27
Brisbane 25 5 30 /27
Perth 10 4 14 /28
Perth 5 3 6 /29
WAN 2 2 2 /30
10
VLSM Allocation Strategy

• Sort the subnet requirements by size, largest
  subnet first.
• For each required subnet, allocate address space
  out of the smallest available subnet
• 128.186.1.0/27
• 128.186.1.32/27
• 128.186.1.64/27
• 128.186.1.96/27
• 128.186.1.128/27
• 128.186.1.160/27
• 128.186.1.192/27
• 128.186.1.224/27

11
VLSM
subnets Hosts available networks
1.0/27 32 1/8 Brisbane
1.32/27 32 1/8 Brisbane
1.64/28 16 1/16 Perth
1.80/29 8 1/32 Perth
1.88/30 4 1/64 WAN
1.92/30 4 1/64
1.96/27 32 1/8
1.128/25 128 1/2
12
Assign network addresses
Brisbane
Perth
13
Task

• Please attempt the task on VLSM posted on module
  web page