1. VLSM Asso.Prof. Dr.Anan Phonphoem Department of Computer Engineering,
Faculty of Engineering, Kasetsart University,
Bangkok, Thailand

2. 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

3. Subnet with VLSM
/24

4. Regular Subnet
/24
7 subnets; The largest subnet needs 60 hosts
If 3 bits for subnet (8 subnets)  5 bits for host (32 hosts)
If 6 bits for host (64 hosts)  2 bits for subnet (4 subnets)

5. Subnet with VLSM Select the biggest first 192.168.10.0/24
/26
/26
/26
/26
/27
/27
/27
/27
Perth KL

6. Subnet with VLSM
/27
/28
/28
/28
/28
/30
/30
/30
/30
Sydney
Singapore
Perth – KL
Sydney – KL
Singapore – KL

7. Subnet with VLSM

8. Assignment 165.23.208.0/20 250 Stations 700 Stations 500 Stations
CPE IE EE
700 Stations
500 Stations
100 Stations

9. Solution The largest subnet needs 700 hosts
CPE IE EE
700 Stations
500 Stations
100 Stations
250 Stations
Solution
/20
/22
/22
/22
/22
1022 Hosts
The largest subnet needs 700 hosts
10 bits for host (1024 hosts)
/22
/23
/23
510 Hosts
/23
/24
/24
254 Hosts
/25
/30
/30
/30 …
2 Hosts
/24
/25
/25
126 Hosts

10. Solution
/24
250 Stations
/20
CPE
/30 IE
/30 EE
700 Stations
500 Stations
100 Stations
/22
/23
/25

11. 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