Subnet Mask

• 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: 192.168.123.132
        • Binary: 11000000.10101000.01111011.1000010
      • Subnet mask: 255.255.255.0
        • Binary: 11111111.111111111.111111111.00000000
      • Network ID: 192.168.123.0
      • Host ID: 0.0.0.132
• 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