HOST PORTIONNETWORK PORTION The problem with subnetting A: 28 hosts B: 60 hosts C: 20 hostsD: 10 hosts E: 2 hosts Network: /24 The largest subnet: 60 hosts Bits required in host portion? 5 bits? = 30 6 bits? = 62 ✔ bits for subnet portion How many subnets? 2 2 = 4 Not enough subnets! Subnets needed: 5 Bits required for subnet portion? 2 bits?2 2 = 4 3 bits?2 3 = 8 ✔ 5 bits are left for host portion How many hosts? = 30 The largest subnet won’t fit!

Subnetting is like slicing a pizza HOST PORTIONNETWORK PORTION / / / / / /26 /25 /26 / / / / /28 HOST PORTIONNETWORK PORTION /28 /26

E D C A B How to VLSM A: 28 hosts B: 60 hosts C: 20 hostsD: 10 hosts E: 2 hosts Network: / /24 /25 / / / / /30 NetNet AddrSizePrefixSubnet MaskAssignable RangeBroadcast A / B / C / D / E /

In reality, VLSM is rarely used as this in corporate networks Address space gets packed and does not allow for easy future expansion Addresses for networks are difficult to memorize Configurations (routing, ACLs, DHCP) are harder to implement and maintain Unnecessary complexity, prone to errors Private address spaces are plentiful to create intuitive addressing schemes: = 10. building number. floor number. host address = 10. campus id. dept id. host address Understanding VLSM is still very useful to Be fluent with IPv4 address arithmetic Understand CIDR and summarization Note