NWF CHAP 5

• Subnetting

  The process of dividing a single address block into multiple logical networks
• How to do Subnetting

  1. Borrow bits from the Host portion of the IP address block

  2. The "borrowed bits" become part of the Network portion

  3. The number of bits to be "borrowed", m, depends on the number of subnets to be created
• Number of Subnets

  2^m ≥ number of subnets
• Routers link up subnets, each interface of a router is connected to a different subnet
• Interconnection between routers is also considered a subnet
• Fixed-length Subnet Mask (FLSM)


  • Assigns the same number of bits in the host portion for any subnet, regardless of the number of hosts in the subnets
  • Results in very wasteful, unused IP addresses
• Variable-Length Subnet Mask (VLSM)

  Allows for more efficient use of IP addresses by assigning different subnet masks to different subnets based on their needs
• How to perform subnetting using VLSM

  1. Divide the address block into subnets based on the number of hosts required in each subnet

  2. Assign the appropriate subnet mask to each subnet

  3. Determine the network ID, valid host addresses, and broadcast address for each subnet
• Fixed-length Subnet Mask (FLSM)

  • Each subnet has equal number of hosts
  • Assigns same number of bits in the host portion for any subnet, regardless of the number of hosts in the subnets
• FLSM results in very wasteful, unused IP addresses
• Variable-Length Subnet Mask (VLSM)

  • Further subnetting a subnet
  • Allows us to divide an IP address block into subnets of different sizes
  • Avoids wasting large numbers of addresses as in FLSM
• How to perform subnetting using VLSM

  1. Re-order the subnets according to number of host addresses needed, in descending order

  2. For each subnet, calculate the number bits needed in the host portion, n, such that (2n-2) ≥ number of host addresses needed

  3. Allocate a block of 2n IP addresses to that subnet. Reserve the first IP address as the Network ID, and last IP address as the Broadcast Address. The rest are valid host addresses

  4. Repeat for each subnet, with an IP address block starting immediately after the Broadcast Address of the previous subnet