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Establishing the subnet mask address

Establishing the subnet mask address
10.3.3 This page provides detailed information about subnet masks and how they are established on a network.


Selecting the number of bits to use in the subnet process will depend on the maximum number of hosts required per subnet. An understanding of basic binary math and the position value of the bits in each octet is necessary when calculating the number of subnetworks and hosts created when bits were borrowed.

The last two bits in the last octet, regardless of the IP address class, may never be assigned to the subnetwork. These bits are referred to as the last two significant bits. Use of all the available bits to create subnets, except these last two, will result in subnets with only two usable hosts. This is a practical address conservation method for addressing serial router links. However, for a working LAN this would result in prohibitive equipment costs.

The subnet mask gives the router the information required to determine in which network and subnet a particular host resides. The subnet mask is created by using binary ones in the network bit positions. The subnet bits are determined by adding the position value of the bits that were borrowed. If three bits were borrowed, the mask for a Class C address would be 255.255.255.224. This mask may also be represented, in the slash format, as /27. The number following the slash is the total number of bits that were used for the network and subnetwork portion.

To determine the number of bits to be used, the network designer needs to calculate how many hosts the largest subnetwork requires and the number of subnetworks needed. As an example, the network requires 30 hosts and five subnetworks. A shortcut to determine how many bits to reassign is by using the subnetting chart. By consulting the row titled ”Usable Hosts”, the chart indicates that for 30 usable hosts three bits are required. The chart also shows that this creates six usable subnetworks, which will satisfy the requirements of this scheme. The difference between usable hosts and total hosts is a result of using the first available address as the ID and the last available address as the broadcast for each subnetwork. Borrowing the appropriate number of bits to accommodate required subnetworks and hosts per subnetwork can be a balancing act and may result in unused host addresses in multiple subnetworks. The ability to use these addresses is not provided with classful routing. However, classless routing, which will be covered later in the course can recover many of these lost addresses.

The method that was used to create the subnet chart can be used to solve all subnetting problems. This method uses the following formula:

Number of usable subnets = two to the power of the assigned subnet bits or borrowed bits, minus two. The minus two is for the reserved addresses of network ID and network broadcast.

(2 power of borrowed bits) – 2 = usable subnets

(23) – 2 = 6

Number of usable hosts = two to the power of the bits remaining, minus two (reserved addresses for subnet id and subnet broadcast).

(2 power of remaining host bits) – 2 = usable hosts

(25) – 2 = 30

The next page will explain how a subnet mask is applied.

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