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IPv4 - Subnetting

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read Forwarding IP Packets - Between Networks before this

subnetting - reduces total number of network numbers that are assigned

when a host wants to send a packet to a certain IP address, first perform bitwise AND between its own subnet mask and the destination IP address:

  • if equal, destination host is on same subnet (physical network) and packet can be delivered directly over the subnet
  • if not equal, packet is sent to a default router

Example Subnetting

consider figure 3.21, where node H1 is sending to node H2

  • H1 ANDs its subnet mask (255.255.255.128) with the address of H2 (128.96.34.139) resulting 128.96.34.128

  • 128.96.34.128 is not equal to H1’s subnet number 128.96.34.0

  • H1 sends packet to router R1

  • R1 checks forwarding table (table 3.7) which contains tuples of <SubnetNumber, SubnetMask, NextHop>

  • to find right entry in forwarding table, the router ANDs the packet’s destination address with the SubnetMask for each tuple entry and compares result to SubnetNumber

    • if entry found, it forwards packet to the NextHop indicated in entry
    • if entry not found, a default route would usually be included in the forwarding table

  • in our scenario, R1 found a matching entry and would forward the datagram to H2 using interface 1