Lecture #2 VLSM Asst.Prof. Dr.Anan Phonphoem Department of Computer Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand

Outline 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 /24

Subnet with VLSM Select the biggest first / / / / / / / / / /27 Perth KL

Subnet with VLSM / / / / / / / / / /30 Sydney Singapore Perth – KL Sydney – KL Singapore – KL

Subnet with VLSM

Assignment CPEIE EE / Stations 500 Stations100 Stations 250 Stations

Solution / / / / / / / / / / / / / / /30 … 1022 Hosts 510 Hosts 254 Hosts 126 Hosts 2 Hosts

Solution CPEIE EE / Stations 500 Stations100 Stations 250 Stations / / / / / /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 ( )