Subnetting the subnet RD-CSY /09

 Company with 200 hosts  Assigned class C network /24  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 ? RD-CSY /09

 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-CSY /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-CSY /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? RD-CSY /09

 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 /22  An ISP can obtain a block of addresses and partition this further to its customers  Say an ISP has /24 address (256 addresses) ◦ He has another customer who needs only 4 addresses from then that block can be specified as /30 RD-CSY /09

Given IP address /24, use VLSM to create a subnetting scheme

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