Subnetting is the process of breaking down a main class A, B, or C network into subnets for routing purposes. A subnet mask is the same basic thing as a netmask with the only real difference being that you are breaking a larger organizational network into smaller parts, and each smaller section will use a different set of address numbers. This will allow network packets to be routed between subnetworks. When doing subnetting, the number of bits in the subnet mask determine the number of available subnets. Two to the power of the number of bits minus two is the number of available subnets. When setting up subnets the following must be determined:

  • Number of segments
  • Hosts per segment

Subnetting provides the following advantages:

  • Network traffic isolation - There is less network traffic on each subnet.
  • Simplified Administration - Networks may be managed independently.
  • Improved security - Subnets can isolate internal networks so they are not visible from external networks.

A 14 bit subnet mask on a class B network only allows 2 node addresses for WAN links. A routing algorithm like OSPF or EIGRP must be used for this approach. These protocols allow the variable length subnet masks (VLSM). RIP and IGRP don't support this. Subnet mask information must be transmitted on the update packets for dynamic routing protocols for this to work. The router subnet mask is different than the WAN interface subnet mask.

One network ID is required by each of:

  • Subnet
  • WAN connection

One host ID is required by each of:

  • Each NIC on each host.
  • Each router interface.

Types of subnet masks:

  • Default - Fits into a Class A, B, or C network category
  • Custom - Used to break a default network such as a Class A, B, or C network into subnets.