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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Subnet Addressing ("Subnetting") Concepts

IP Default Subnet Masks For Address Classes A, B and C 1 2 3 4 IP Subnet Identifiers, Subnet Addresses and Host Addresses

It's important to understand what default subnet masks are and how they work. A default subnet mask doesn't really represent subnetting, however, since it is the case where we are assigning zero bits to the subnet ID. To do “real” subnetting we must dedicate at least one of the bits of the pre-subnetted host ID to the subnet ID, as shown in the example in the topic that introduced subnet masks.

Since we have the ability to customize our choice of dividing point between subnet ID and host ID to suit the needs of our network, this is sometimes called customized subnetting. The subnet mask that we use when creating a customized subnet is, in turn, called a custom subnet mask. The custom subnet mask is used by network hardware to determine how we have decided to divide the subnet ID from the host ID in our network.

The key decision in customized subnetting is how many bits to take from the host ID portion of the IP address to put into the subnet ID. Recall that the number of subnets possible on our network is two to the power of the number of bits we use to express the subnet ID, and the number of hosts possible per subnet is two to the power of the number of bits left in the host ID (less two, which I will explain later in this topic).

Thus, the decision of how many bits to use for each of the subnet ID and host ID represents a fundamental trade-off in subnet addressing:

• Each bit taken from the host ID for the subnet ID doubles the number of subnets that are possible in the network.
  
  
• Each bit taken from the host ID for the subnet ID (approximately) halves the number of hosts that are possible within each subnet on the network.

Let's take a brief example or two to see how this works. Imagine that we start with a Class B network with the network address 154.71.0.0. Since this is Class B, 16 bits are for the network ID (154.71) and 16 are for the host ID. In the default case there are no subnets (well, one “subnet” that is the whole network) and 65,534 hosts total. To subnet this network, we have a number of choices:

1. We can decide to use 1 bit for the subnet ID and 15 bits for the host ID. If we do this, then the total number of subnets is 2¹ or 2: the first subnet is 0 and the second is 1. The number of hosts available for each subnet is 2¹⁵-2 or 32,766.
   
   
2. We can use 2 bits for the subnet ID and 14 for the host ID. In this case, we double the number of subnets: we now have 2² or 4 subnets: 00, 01, 10 and 11 (subnets 0, 1, 2 and 3). But the number of hosts is now only 2¹⁴-2 or 16,382.
   
   
3. We can use any other combination of bits that add up to 16, as long as they allow us at least 2 hosts per subnet: 4 and 12, 5 and 11, and so on.

IP Default Subnet Masks For Address Classes A, B and C 1 2 3 4 IP Subnet Identifiers, Subnet Addresses and Host Addresses