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TechKnowSurge Catalog Interactives Subnet Borrowing Capacity Calculator
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Subnet Borrowing Capacity Calculator

Borrow bits from a Class A, B or C network and type how many subnets you get and how many hosts each one holds.

⚑ Complete this interactive to capture a CTF flag worth 5 points.

About this interactive

Subnetting moves the line between the network and host portions of an address to the right. Each bit you borrow from the host side becomes a network bit, so borrowing b bits gives 2^b subnets: one bit gives 2, eight bits give 256. The bits left on the host side set the size of each subnet: h host bits give 2^h addresses, and two of them, the all-zeros network address and the all-ones broadcast address, are not given to hosts, so 2^h - 2 are usable. The class default is where you start counting. Class A's default is /8, Class B's /16 and Class C's /24, so 10.0.0.0 subnetted to /24 has borrowed 16 bits and has 256 addresses (254 usable) in each of its 65,536 subnets. 172.16.0.0 with one bit borrowed is /17: 2 subnets of 32,768 addresses, 32,766 usable. Each extra bit borrowed doubles the subnets and halves the addresses in each one. This set stops at /30, the smallest subnet with two usable hosts. A /31 is the special case RFC 3021 allows on point-to-point links, where both of its two addresses are usable, and a /32 is a single host; neither follows the minus-two rule. Type counts in digits (commas are fine). Anything you miss shows the answer and how to work it out. Seven of the twenty-one questions are drawn each run.

What you'll learn

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

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).
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.
Host Bits
The portion of an IPv4 address that identifies individual devices within a network segment, as determined by the subnet mask.
Prefix Length
The number of consecutive 1-bits in a subnet mask, written after a slash (e.g., /24), that defines how many bits of an IP address represent the network portion versus the host portion. A /24 prefix means the first 24 bits identify the network, leaving 8 bits for host addresses.
Classful Addressing
An early IPv4 addressing scheme that divided the address space into fixed classes (A, B, and C for hosts; D for multicast; E reserved) based on the leading bits of the address, with each class defining a fixed boundary between the network and host portions. Classful addressing was replaced by CIDR because it wasted large blocks of address space.

Topics

Subnetting Bits Borrowed Usable Hosts Interactive Calculation

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