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Module 1: Introduction to Classless Routing

Module 1: Introduction to Classless Routing. CCNA 3 version 3.1. The Addressing Crisis. IPv4 (32 bit address space) – exhausting address space Short Term Solutions: Subnetting (1985) VLSM (1987) Classless Interdomain Routing (1993) Private IP addresses Network Address Translation (NAT)

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Module 1: Introduction to Classless Routing

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  1. Module 1:Introduction to Classless Routing CCNA 3 version 3.1

  2. The Addressing Crisis • IPv4 (32 bit address space) – exhausting address space • Short Term Solutions: • Subnetting (1985) • VLSM (1987) • Classless Interdomain Routing (1993) • Private IP addresses • Network Address Translation (NAT) • Ultimate Solution: • IPv6 (128 bit address space) • Allows for 340,283,366,920,938,463,374,607,431,768,211,456 possibilities

  3. What is VLSM and how does it work? • VLSM is… • Variable Length Subnet Masking • One answer to depletion of address space • Helps keep routing tables lean • With VSLM a network administrator can… • use more than one subnet mask within the same network address space • use a long mask on networks with few hosts, and a short mask on subnets with many hosts • Often referred to as "subnetting a subnet“ • can be used to maximize addressing efficiency

  4. A VLSM Example /30 is used on the serial links since only two addresses are needed /27 is used on the LANs for host addressing (allows for 30 usable hosts per subnet)

  5. A Waste of Space • Take the Class C address of 192.168.187.0 and a /27 subnet mask (3 bits borrowed for subnetting) • Though it is not good practice, you may use the first and last subnet with VLSM (subnet zero and the all-ones subnet) • Let’s say this company decides to use the subnet zero so it has 8 usable subnets with 30 hosts per subnet

  6. A Waste of Space Cont’d… • This company must use three of the subnets to assign to the 3 WAN links (which only need two IP addresses each) • That’s a waste of 28 addresses per subnet!

  7. Using VLSM on the Point-to-Point Links • Sub-subnets

  8. The Result… • Notice the /27 bit masks for the LANs and the /30 for the serial links.

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