Subnet Mask

Default IPv4 subnet masks

  • Used so devices can determine which part of an IP address is network ID and which part is host ID
    • Number of 1s in subnet mask --> number of bits belonging to the network ID
    • Number of 0s in subnet mask --> number of bits belonging to the host ID
      • Example:
        • IP address:
          • Binary: 11000000.10101000.01111011.1000010
        • Subnet mask:
          • Binary: 11111111.111111111.111111111.00000000
        • Network ID:
        • Host ID:
  • Useful for sending transmissions to another host
    • Compares bits in its own network ID to bits in the network ID of destination host
    • If bits match:
      • Destination host on same network
      • Sends transmission directly to destination host
    • If bits don't match:
      • Destination host on different network
      • Sends transmission directly to default gateway on the network

Subnet Mask Tables

  • Class A, Class B, and Class C networks can be subnetted (subnetting)
    • Each class --> different number of host info bits usable for subnet info
    • Varies depending on network class and the way subnetting is used

Variable Length Subnet Mask (VLSM)

  • Allows subnets to be further subdivided
    • Smaller groupings until each subnet is about the same as the necessary IP address space
  • Referred to as subnetting a subnet

Networking Computer Science