Subnet Addressing (Contd)

• Internal routers & hosts use subnet mask to identify “subnet ID” and route packets between “subnets” within the “network”.
• Eg: Mask: 255.255.255.0 => subnet ID = 8 bits with upto 62 hosts/subnet
• Route table lookup:
  • IF ((Mask[i] & Destination Addr) = =
    Destination[i])  Forward to NextHop[i]
• Subnet mask can end on any bit.
• Mask must have contiguous 1s followed by contiguous zeros. Routers do not support other types of masks.

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