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VLSM Variable Length subnet masks. Subnetting the subnet. 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)
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VLSMVariable Length subnet masks Subnetting the subnet RD-CSY2001-2008/09
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 ? RD-CSY2001-2008/09
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 RD-CSY2001-2008/09
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 RD-CSY2001-2008/09
Task 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? RD-CSY2001-2008/09
VLSM - example RD-CSY2001-2008/09
CIDR: Classless Inter-Domain Routing • Address format <IP address/prefix P>. • The prefix denotes the upper P bits of the IP address. • Can be used to specify arbitrary blocks of addresses • Say an ISP has 192.5.48.0, 192.5.49.0,192.5.50.0,192.5.51.0 then • the IP address advertised will be 192.5.48.0/22 • 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 200.8.4.24 then that block can be specified as 200.8.4.24/30 RD-CSY2001-2008/09
Task Given IP address 204.15.5.0/24, use VLSM to create a subnetting scheme RD-CSY2001-2008/09
Additional Tasks Please try other questions on VLSM, available on module web page RD-CSY2001-2008/09