31 bit prefixes (RFC 3021)

About this video

Subnetting requires borrowing bits from the host portion of an IP address, and the number of bits borrowed directly controls the tradeoff between available networks and available hosts per network. A subnet reference table maps each possible bit-borrowing value to its corresponding subnet mask, network count, and host count, making it possible to evaluate options systematically. The correct number of bits is determined by finding the smallest value that simultaneously meets or exceeds the required number of networks and the required number of hosts. When no single line satisfies both constraints within the available address space, that addressing scheme is not viable for the given requirements. In many real-world scenarios, more than one bit-borrowing value will satisfy the minimum requirements, which introduces a design decision. If host count is expected to grow, borrowing fewer bits preserves more host addresses per subnet. If the number of network segments is expected to expand, borrowing more bits provides that room. When growth direction is uncertain, a middle-value option balances flexibility across both dimensions. This reasoning applies consistently whether working with Class C or Class B address space, with Class B simply offering a wider range of valid configurations to evaluate.

What you'll learn

Determine the number of bits to borrow when subnetting a Class C or Class B address to satisfy specified network and host requirements
Evaluate trade-offs between network and host capacity when multiple valid subnet boundary options exist

What's covered

Determining Bits to Borrow

Why Borrow Bits
- Networks Needed
- Hosts Needed
Reference Tables
- Bits Borrowed vs Networks
- Bits Borrowed vs Hosts
- Subnet Mask Shown
Finding the Line
- Meet Network Requirement
- Meet Host Requirement
- Impossible Scenarios (X)
Multiple Valid Options
- Grow Host Flexibility
- Grow Network Flexibility
- Balance Both Sides
Class B vs Class C
- Same Selection Process
- Different Range Values

Aligned to

Cisco CCNA

1.6 Configure and verify IPv4 addressing and subnetting

CompTIA Network+

1.7 Given a scenario, use appropriate IPv4 network addressing

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.
Bits Borrowed: The number of bits taken from the host portion of an IP address to create additional subnet identifiers, directly controlling the trade-off between number of subnets and number of hosts per subnet.
Network Capacity: The maximum number of subnets available given a specific number of bits borrowed, calculated as 2 raised to the power of borrowed bits.
Host Capacity: The maximum number of usable host addresses per subnet, calculated as 2 raised to the power of remaining host bits minus 2 (reserving the network and broadcast addresses).

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