DEMO: Subnetting Class B — TechKnowSurge

About this video

A Class B address in its default state uses CIDR notation of /16 and a subnet mask of 255.255.0.0, producing a single network with 65,534 usable hosts — calculated as 2 to the 16th power minus 2. Borrowing bits from the host portion shifts that balance, with each borrowed bit doubling the number of available networks while halving the host space. Borrowing one bit, for example, produces a /17 with a subnet mask of 255.255.128.0, yielding two networks and 32,766 usable hosts per network. This pattern continues incrementally across all valid borrowing combinations until the point at which too few host bits remain to support any usable addresses, making that configuration impractical. Borrowing 15 bits, the outer limit of this range, falls into that category and is marked not applicable. The complete reference table produced by working through every valid Class B subnet configuration serves as a practical tool for subnetting tasks, giving network professionals a reliable guide to network counts, host counts, and subnet masks across the full Class B subnetting spectrum.

What you'll learn

Calculate the number of subnets, hosts per subnet, and usable hosts resulting from borrowing variable bits from a Class B IPv4 address
Construct a Class B subnetting reference table using CIDR notation and subnet masks from /16 through /30

What's covered

Subnetting a Class B Address

Default Class B Settings
- CIDR Notation /16
- Subnet Mask 255.255.0.0
- One Network
- 65,534 Usable Hosts
Borrowing Bits
- One Bit Borrowed /17
  - Mask 255.255.128.0
  - Two Networks
  - 32,766 Usable Hosts
- Continuing to Borrow Bits
  - Expanded Reference Table
- 15 Bits Not Applicable
  - No Usable Hosts
Subnetting Reference Table
- Networks vs Hosts Tradeoff
- Use for Future Subnetting

Aligned to

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing.

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting.

Cisco CCST Networking

2.2 Identify IPv4 addresses and subnet formats.

Key terms

Subnet: A logical subdivision of an IP network, used to organize and segment network traffic.
Subnet Mask: A 32-bit number that divides an IP address into network and host portions.
IP Address: A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Class B Network: An IPv4 address class with a default subnet mask of 255.255.0.0 (/16), supporting up to 65,534 usable hosts on a single network.
Classless Inter-Domain Routing: A method for allocating IP addresses and expressing routing prefixes that replaces the older classful addressing scheme by using variable-length prefix notation (e.g., /24). CIDR enables more efficient use of IPv4 address space and reduces the size of internet routing tables through route aggregation.
Usable Hosts: The number of valid host addresses within a subnet, calculated by subtracting two from the total host count to exclude the network and broadcast addresses.

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