Subnet Mask - Explained

The video begins by defining what an IP address is: a unique identifier for devices on a network, specifically an IPv4 address, which is 32-bit numeric, written as four numbers separated by periods, with each group (octet) ranging from 0-255. An IP address has two parts: a network address and a host address. The subnet mask is introduced as the tool to differentiate between these two parts by masking the network portion.

Computers and networks understand IP addresses and subnet masks in binary format (1s and 0s). The video explains how to convert decimal IP addresses and subnet masks into 8-bit binary octets using a chart where each bit position has a specific value (128, 64, 32, 16, 8, 4, 2, 1). Examples are provided for converting numbers like 192, 168, and 1 into their binary equivalents.

Once the IP address and subnet mask are in binary form, the subnet mask's 1s indicate the network portion of the IP address, and the 0s indicate the host portion. Examples demonstrate how lining up the binary forms of an IP address and its subnet mask clearly shows which bits belong to the network and which to the host.

The video explains why IP addresses have both network and host parts: for manageability and to break down large networks into smaller sub-networks, a process called subnetting. This prevents network congestion caused by excessive broadcast traffic in a single large network. Routers separate these subnetworks, ensuring broadcasts are contained within their respective subnets.

A business scenario is used to illustrate subnetting. If a business with 12 computers wants to separate them into three departments, they need to change the default subnet mask. Subnetting is achieved by borrowing bits from the host portion of the IP address, converting them into network bits, to create more networks. The video shows how borrowing bits increases the number of available networks while decreasing the number of hosts per network.

IP addresses and subnet masks come in five classes (A-E), with A, B, and C being for commercial use. The class determines the number of hosts an organization can support, ranging from millions (Class A) to hundreds (Class C). Finally, the video introduces Classless Inter-Domain Routing (CIDR) or slash notation as a shorthand for writing subnet masks, indicating the number of 1s in the binary subnet mask.