Skip to main content

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.

Comments

Post a Comment

Popular posts from this blog

OSI layers / Peer-to-peer communications / TCP/IP model

OSI layers 2.3.4 This page discusses the seven layers of the OSI model. The OSI reference model is a framework that is used to understand how information travels throughout a network. The OSI reference model explains how packets travel through the various layers to another device on a network, even if the sender and destination have different types of network media. In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. - Dividing the network into seven layers provides the following advantages: • It breaks network communication into smaller, more manageable parts. • It standardizes network components to allow multiple vendor development and support. • It allows different types of network hardware and software to communicate with each other. • It prevents changes in one layer from affecting other layers. • It divides network communication into smaller parts to make learning it easier to understand. In the foll...
Post a Comment
Read more

PC Basic...

• Backplane – A backplane is an electronic circuit board containing circuitry and sockets into which additional electronic devices on other circuit boards or cards can be plugged; in a computer, generally synonymous with or part of the motherboard. • Network interface card (NIC) – An expansion board inserted into a computer so that the computer can be connected to a network. • Video card – A board that plugs into a PC to give it display capabilities. • Audio card – An expansion board that enables a computer to manipulate and output sounds. • Parallel port – An interface capable of transferring more than one bit simultaneously that is used to connect external devices such as printers. • Serial port – An interface that can be used for serial communication in which only one bit is transmitted at a time. • Mouse port – A port used to connect a mouse to a PC. • USB port – A Universal Serial Bus connector. A USB port connects devices such as a mouse or printer to the computer ...
Post a Comment
Read more

Windowing

Windowing 11.1.5 This page will explain how windows are used to transmit data. Data packets must be delivered to the recipient in the same order in which they were transmitted to have a reliable, connection-oriented data transfer. The protocol fails if any data packets are lost, damaged, duplicated, or received in a different order. An easy solution is to have a recipient acknowledge the receipt of each packet before the next packet is sent. If a sender had to wait for an ACK after each packet was sent, throughput would be low. Therefore, most connection-oriented, reliable protocols allow multiple packets to be sent before an ACK is received. The time interval after the sender transmits a data packet and before the sender processes any ACKs is used to transmit more data. The number of data packets the sender can transmit before it receives an ACK is known as the window size, or window. TCP uses expectational ACKs. This means that the ACK number refers to the next packet that is...
Post a Comment
Read more