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DEMO: Subnetting Class A

Class A subnetting follows the same bit-borrowing process used for Class B and Class C addresses, scaled up to handle a significantly larger number of bits across the full address space.

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About this video

Class A subnetting uses the same core process as Class B and Class C subnetting: bits are borrowed from the host portion of the address to create additional network segments. Because Class A addresses have far more host bits available, the resulting reference chart is more extensive, but the underlying logic remains consistent. The default subnet mask for a Class A address yields a single network with an enormous host capacity, and each bit borrowed from that space doubles the network count while cutting available hosts per network in half. Usable host counts are calculated by subtracting two from the raw total to account for the network and broadcast addresses. A complete subnetting chart for Class A addresses spans the full range of borrowable bits, including all eight bits of the final octet. Working through this chart — calculating subnet masks, network counts, and usable host totals for each borrowing increment — reinforces the patterns that appear across all address classes. This reference table is a critical resource for practical subnetting exercises and real-world network design calculations.

What you'll learn

Complete a Class A subnetting reference table by calculating subnet masks, number of networks, and usable host counts for each possible borrowed-bit increment
Identify the pattern of subnet mask values, network counts, and host counts that results from incrementally borrowing bits across all three variable octets of a Class A address

What's covered

Class A Subnetting

Borrowing Bits
- Same Process as B and C
- More Bits Involved
Subnet Mask Chart
- Zero Bits Borrowed
  - Default Subnet Mask
  - One Network
- One Bit Borrowed
  - 255.128.0.0
  - Two Networks
- Continue Filling Chart
Hosts Calculation
- Subtract Two for Usable
- Hosts per Network
Chart Used for Subnetting
- Reference for Later Lessons

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 A Address: An IP address class with a default subnet mask of 255.0.0.0, reserving the first octet for the network portion and the remaining three octets for hosts, supporting a very large number of hosts per network.
Borrowed Bits: Host bits reallocated to the network portion of a subnet mask to create additional subnets, reducing the number of available hosts per subnet.
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.

Transcript

What does it look like when we start borrowing bits from a Class A address? These charts are going to come in handy when we start subnetting, so let's do our last one for our Class A address, so that way we can bring this into when we're doing our subnetting.

The process is just the same as we did with a Class C and the Class B, so you should start seeing the patterns come up. The pattern is going to be similar with a Class A as well — it's just that we're dealing with quite a few more bits.

Filling out the Class A table

I've already started filling out this table. You can see that if we borrow zero bits, there's the default subnet mask right there. It just renders us one network, and it gives us a ton of different hosts — the usable hosts is subtract two from there. Then we went through: if we borrowed one bit from this, we would get a 255.128.0.0, we get two networks, and here's the number of hosts that we get from it.

Go ahead and continue to fill out this chart, and then double check your answers with the answers up on the screen.

Almost done with this. This is the first part of the Class C, but we do have one more octet to do. Here is our last octet, or the last eight bits that we can borrow. Fill out the rest of this, and then check your work against the answers to see how well you did.

These tables we're going to use when we get to actually subnetting everything out.

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