© 2000, Cisco Systems, Inc. 1-1 VLSM 2 2 Issues with IP Addressing • IP addressing crisis • As early as 1992, the IETF identified two specific concerns: – IP address exhaustion – Routing table growth U N I V E R S I T Y U N I V E R S I T Y Internet 3 3 IP Addressing Solutions • Short term to extension to IPv4 – Subneting masking: RFCs 950, 1812 – Variable-length subnet masks: RFC 1812 – Classless interdomain routing: RFCs 1518, 1519, 2050 – Address allocation for private Internets: RFC 1918 – Network Address Translation: RFC 1631 – Route summarization: RFC 1518 • Ultimate solution: IPv6 128-bit address space 4 4 172.16.14.32/27 172.16.14. 64/27 172.16.14.96/27 What is VLSM ? • Subnet 172.16.14.0/24 is divided into smaller subnets (Sub- subnet): – Subnet with one mask at first (/27) – Further subnet one of these subnets not used elsewhere (/30) C B A HQ 172.16.1.0/24 172 .16. 2.0/ 24 HQ 172.16.0.0/16 172.16.14.136/30 172.16.14.132/30 172.16.14.140/30 5 5 A waste of space 6 6 When to use VLSM? 7 7 Calculating subnets with VLSM 8 8 Calculating subnets with VLSM (2) Derived from the 172.16.32.0/20 Subnet 172.16.33.0/30 172.16.32.0/26 172.16.32.64/26 172.16.32.128/26 172.16.32.192/26 26-Bit Mask (62 Hosts) Derived from the 172.16.33.0/26 Subnet 30-Bit Mask (2 Hosts) 172.16.33.12/30 172.16.33.8/30 172.16.33.4/30 9 9 Exercise Calculating subnets with Exercise Calculating subnets with VLSM VLSM 60 Hosts 12 Hosts 12 Hosts Using VLSMs, define appropriate subnets for addressing the networks using 192.168.10.0/24. 28 Hosts KL Sydney Perth Singapore 10 10 Exercise Calculating subnets with VLSM 60 Hosts 12 Hosts 12 Hosts 192.168.10.0/24 KL Sydney Perth Singapore 192.168.10.64/27 192.168.10.128/30 192.168.10.132/30 192.168.10.136/30 28 Hosts 192.168.10.0/26 192.168.10.96/28 192.168.10.112/28 [...]... with VLSM 192.168.48.0/24 192.168.49.0/24 192.168.50.0/24 192.168.51.0/24 192.168.52.0/24 192.168.53.0/24 192.168.54.0/24 192.168.55.0/24 192.168.56.0/24 192.168.57.0/24 192.168.63.0/24 Perth 19 2 1 68 48 0 /2 2 Sydney 192.168.48.0/20 KL 192.168.52.0/22 Upstream Upstream provider provider 1 /2 0 56 68 1 92 1 Singapore Route summarization reduces routing table size by aggregating routes to multiple