Lecture #2 VLSM

Asst.Prof. Dr.Anan Phonphoem

Department of Computer Engineering,

Faculty of Engineering, Kasetsart University,

Bangkok, Thailand

Outline

VLSM

VLSM

Variable-length subnet mask Classful allows only one subnet in a network

> one subnet in an autonomous system Maximizing the use of address (Subnet Zero) “Subnetting a Subnet” Routing that support VLSM

OSPF, Integrated IS-IS, EIGRP, RIPv2, and static routing

Subnet with VLSM

192.168.10.0/24

Subnet with VLSM

Select the biggest first 192.168.10.0/24

192.168.10.0/26 192.168.10.64/26 192.168.10.128/26 192.168.10.192/26

192.168.10.64/26 192.168.10.64/27 192.168.10.96/27 192.168.10.128/27 192.168.10.192/27

Perth

KL

Subnet with VLSM

192.168.10.96/27 192.168.10.96/28 192.168.10.112/28 192.168.10.224/28 192.168.10.240/28

192.168.10.224/28 192.168.10.128/30 192.168.10.132/30 192.168.10.136/30 192.168.10.140/30

Sydney

Singapore

Perth – KL

Sydney – KL

Singapore – KL

Subnet with VLSM

Assignment

CPE IE EE

165.23.220.0/20

700 Stations 500 Stations 100 Stations

250 Stations

Solution

•165.23.220.0/20•165.23.208.0/22•165.23.212.0/22•165.23.216.0/22•165.23.220.0/22

•165.23.212.0/22•165.23.212.0/23•165.23.214.0/23

•165.23.214.0/23•165.23.214.0/24•165.23.215.0/24

•165.23.215.0/24•165.23.215.0/25•165.23.215.128/25

•165.23.215.128/25•165.23.215.128/30•165.23.215.132/30•165.23.215.136/30•…

1022 Hosts

510 Hosts

254 Hosts

126 Hosts

2 Hosts

Solution

CPE IE EE

165.23.220.0/20

700 Stations 500 Stations 100 Stations

250 Stations

165.23.208.0/22

165.23.214.0/24

165.23.215.128/30 165.23.215.132/30

165.23.212.0/23 165.23.215.0/25

Route Aggregation

Route summarization Supernetting Possible only if routers of a network use a

classless routing protocol (OSPF or EIGRP) Carry 32-bit IP address and bit mask in the

routing updates Share the same high-order bits

Routing Protocols

Autonomous systems (AS) generally administered by a single entity Each AS has a routing technology which can

differ from other autonomous systems Interior Gateway Protocol (IGP)

Routing in the AS Exterior Gateway Protocol (EGP)

transfer routing information between autonomous systems

RIP

work as an IGP in a moderate-sized AS RIPv1 is classful IGP distance vector protocol Use hop count as a metric Route update is 30 sec (Broadcast) load balancing upto 6 paths

RIPv2

allows it to send out subnet mask information with the route update

provides for authentication in its updates multicasts routing updates (224.0.0.9)