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Automatic Subnet Numbering

Automatic Subnet Numbering. Hilarie Orman. Layer Addressing Characteristics. Layer 2 Unique, uncorrelated addresses Linear strings of nodes Join points Spanning tree Layer 3 Address blocks grouped by contiguous bits Mesh of routers Configured topology over layer 2

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Automatic Subnet Numbering

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  1. Automatic Subnet Numbering Hilarie Orman

  2. Layer Addressing Characteristics • Layer 2 • Unique, uncorrelated addresses • Linear strings of nodes • Join points • Spanning tree • Layer 3 • Address blocks grouped by contiguous bits • Mesh of routers • Configured topology over layer 2 • Universal address space

  3. X Routers and Subnetswithout IP addrs, no routing X X X X

  4. IP Address Subnet number Fixed prefix ARIN or non-routable Host number DHCP

  5. X SubnetNumbers SUBNET 7 SUBNET 1 X SUBNET8 SUBNET 5 SUBNET 2 SUBNET 9 X X SUBNET 3 SUBNET 6 X SUBNET 4

  6. Requirements • Support initialize, join, leave, partition, rejoin • Utilize IPv4 addresses efficiently • Solve v6 and v4 assignments • Cooperate with DHCP • Minimal interference with ongoing end node connections • Be secure

  7. Central Authority Request Subnet Number X Reply Subnet Number Single point of failure --- ameliorate with elections

  8. Distributed Agreement Protocol X X X Router 1 needs 3 subnet numbers X Router 2 needs 2 subnet numbers X

  9. Distributed Protocol Problems • Might be slow at startup • Partitions due to failure require renumbering • Partition joins as part of network growth would be complicated • Authentication … via secure multicast keying?

  10. X X Network Partition / Nodes Join SUBNET 7 SUBNET 1 SUBNET 10 X SUBNET8 SUBNET 2 X SUBNET 9 X SUBNET 3 SUBNET 10 X SUBNET 4

  11. X Node Mobility SUBNET 1 Prefix | subnet 1 | hostnum X SUBNET 2 SUBNET 4 X SUBNET 3 Prefix | subnet 4 | hostnum’

  12. Address Utilization • Mobility makes subnet size difficult to predict • Simplest assignment method divides address space into N subnets of equal size • Must overprovision and waste • Dynamic subnet size adjustment • Difficult to synchronize

  13. Automatic Numbering at Layer 3Main Points • No end node mobility without mobile IP • Partition/rejoin might require reassigning end node addresses • Breaks connections • Sizing subnets is awkward

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