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TechKnowSurge Catalog Videos DEMO: Subnetting Class C
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DEMO: Subnetting Class C

Class C subnetting covers how borrowing host bits creates custom subnet masks, increasing the number of available networks while reducing the number of usable hosts per network.

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About this video

Class C networks carry a default subnet mask of 255.255.255.0, which designates the final octet entirely for host addressing. This yields a single network with 256 total host addresses, of which 254 are usable once the network address and broadcast address are excluded. When subnetting is applied, bits are borrowed from the host portion of the address, shifting the subnet mask boundary and creating multiple subnetworks from a single Class C block. Each bit borrowed follows a predictable mathematical pattern. Borrowing one bit produces a /25 mask (255.255.255.128), yielding two networks with 126 usable hosts each. Borrowing two bits produces a /26 mask (255.255.255.192), yielding four networks with 62 usable hosts each. This pattern continues, with the number of networks calculated as 2 to the power of borrowed bits and usable hosts calculated as 2 to the power of remaining host bits minus 2. Borrowing seven bits from a Class C host octet is not practical in production environments because it leaves no usable host addresses. A /32 mask, while not a true network designation, does appear in practice as a notation for a single host address. All other subnet configurations within the Class C range are commonly encountered and represent the core calculations network professionals apply when designing and segmenting IP address spaces.

What you'll learn

What's covered

Subnetting Class C Addresses

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.
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.
Network Address
The first address in an IP network range that identifies the network itself and cannot be assigned to a host. It is formed when all host bits are set to zero.
Broadcast Address
The last address in an IP subnet range, formed when all host bits are set to one, used to send a single packet to every device on that network simultaneously.
Bit Borrowing
The practice of taking bits from the host portion of a classful default subnet mask to create additional network subdivisions, extending the prefix length.

Transcript

A Class C network would have a default mask of 255.255.255.

What we can do is we can start by filling in the graph below. If we don't borrow any bits, our mask would be 255.255.255.0. The number of networks that we can have is just one; it's just whatever is specified by those network bits. The number of hosts we have available to us is this last octet, so that's 2 to the eighth power, which we know is 256. And we do have to subtract 2 from that, because we have the network address and the broadcast address, so actual usable is 254, and total host addresses is 256.

Borrowing One Bit

Now let's say we want to create a custom subnet mask with this. What we're going to do is we're going to just borrow one bit, so I'm looking at this next row in this table. I'm going to borrow one bit. The CIDR notation for that is a slash 25. The subnet mask for that would be 255.255.255, which is the default mask there, and the last octet we could figure out if we had a 1 here. I've got this little cheat sheet over here on the side, and if we just borrow one bit that would be in the 128th position there, so it would be dot 128.

What this would give us is one bit could be a zero or a one, so that's two different networks that we've just created. So number of networks, we've got two networks that we've created there. Number of hosts, we have seven bits left for hosts, so two to the seventh power, and that's going to be 128. So number of hosts is 128. Usable, we've got to subtract two out of there, so 126 is the answer there.

Borrowing Two Bits

Go on to the next one. We're actually going to borrow two bits here, so I'm going to remove that. We'll draw the line right here. If I'm borrowing two bits, we've got 255.255.255 dot, and then this is 192 for two bits, so 192. The number of networks we have here is 2 to the 2 power, that's four, so we've got four networks there. Number of hosts, we've got two to the sixth power, so that is 64 hosts there. Number usable would be 62.

Filling Out the Rest of the Chart

So you see where I'm going with this, and what you can do is you can fill out the rest of this chart, and then we'll come together and see what the answers are.

Here are the addresses. I do want to point out here, I did put not applicable for this borrowing seven bits on a Class C. It doesn't really make sense, and the reason why it doesn't make sense is because you wouldn't have any usable hosts available to you, so you won't really see this. I don't know that I've ever seen a slash 31 out there.

I have, however, seen a slash 32, and that just means the address itself. So if you have just a single address, or it's specifying just a single address, I have seen a slash 32 before. Usually it's not a network address, it's a single host address, and so occasionally you will see that. But the rest of these, definitely I've seen quite a few out there using all the rest of these.

Check it against your answers, make sure you got everything correctly. This is one of those things that now you can use the table that you created for future assignments. We're actually going to reference this table a lot, it's going to come up a lot, so when we do our subnetting exercises this is a great table to use as a little cheat sheet in those exercises.

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