VLSM Variable Length subnet masks

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Transcript VLSM Variable Length subnet masks

Subnetting the subnet


    Company with 200 hosts Assigned class C network Want 6 different LANs Subnet class C network using mask (/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 ?


 ◦

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 RD-CSY2001-2008/09

 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 RD-CSY2001-2008/09

Question  If an ISP has a 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?




     Address format .

The prefix denotes the upper P bits of the IP address.

◦ ◦ Can be used to specify arbitrary blocks of addresses Say an ISP has,,, then the IP address advertised will be An ISP can obtain a block of addresses and partition this further to its customers ◦ Say an ISP has 200.8.4/24 address (256 addresses) He has another customer who needs only 4 addresses from then that block can be specified as RD-CSY2001-2008/09

Net A: 14 HOSTS Router A Net C: 2 HOSTS Net D: 7HOSTS Router B Net E: 28 HOSTS Net B: 28 HOSTS Given IP address, use VLSM to create a subnetting scheme RD-CSY2001-2008/09

Please try other questions on VLSM, available on module web page RD-CSY2001-2008/09