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TechKnowSurge Catalog Videos DEMO: Subnetting Example A-1
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DEMO: Subnetting Example A-1

Class A subnetting follows the same process as other address classes, using borrowed bits to meet network size requirements within the 10.0.0.0 range and its default mask of 255.0.0.0.

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About this video

Class A subnetting operates on the same fundamental principles as subnetting in other address classes, with the primary difference being the much larger address space involved. The default subnet mask for a Class A address is 255.0.0.0, leaving three full octets available for host addressing, which means significantly more bits can be borrowed to create subnets at scale. The 10.0.0.0 range is the most commonly encountered Class A block in professional environments, making familiarity with it a practical advantage. When the requirement is to support 15,000 networks, borrowing 14 bits satisfies that demand by providing over 16,000 total subnets. The resulting custom subnet mask is 255.255.252.0, and each subnet supports 1,022 usable hosts after accounting for the network and broadcast addresses. While enumerating all subnets in a Class A block would be impractical, the same incremental process used for smaller address classes applies here, and targeted questions such as identifying a specific subnet by number are answered using that same systematic approach.

What you'll learn

What's covered

Class A Subnetting Example

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
CompTIA A+ Core 1
2.6 Compare and contrast common network configuration concepts.

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 Network
An IPv4 address class using the range 1–126 in the first octet, with a default subnet mask of 255.0.0.0, providing a large number of host addresses.
Classless Inter-Domain Routing
CIDR
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.
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.

Transcript

Time for an example. We're not going to go through all of it, because once again it's very similar to everything that we have done, but just to show you that a class A is no different, let's jump into an example.

This is no different, although it's so big now that it falls off the end of my screen. I wasn't able to make it big enough for this purpose, but we could do the same things.

Let's say I'm going for 15,000 networks here, so I need to find something that's going to fit 15,000, and the 16,000 will do the trick. So I could draw the line right there. It's looking for 851 networks, so that is right here. So everything works out with the line being right there.

What's the class? This is the 10 range. Good chances that you'll work with the 10 range in the future. So this is a class A. Then the default subnet mask would be 255.0.0.

The number of borrowed bits for this is, well, we're going to borrow 14 bits, so 14 bits for that. The custom subnet mask is 255.255, let's see, this would be 252, so 252.0.

Total number of subnets, we've got over 16,000 subnets. Total hosts, we've got over a thousand hosts for this. Usable hosts specifically, it's 1,024. This doesn't have the two removed from it, so 1,022.

So there you have it. That's how we'd figure that out, and then we would go through that same process. You can see that if we would do all 16,000 networks, that would take a ton of time. But you could just do however many, if it's asking. Sometimes they'll ask for, like, what's the 12th network, and you'd have to do it up until the 12th network and then write it all out.

So there you have it, that is how to subnet, and that's a class A.

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