Videos/Introduction to IP Subnetting
Introduction to IP Subnetting
26 min lesson, taught by CISNET instructor Mani Poopal (CCIE).
In this lesson, CCIE instructor Mani Poopal explains IP subnetting for CCNA students, starting from what subnetting is (dividing one large network into smaller networks) and why we do it (more control and better security). You will learn to read the subnet mask to tell whether a network is subnetted, how borrowing bits creates subnets, and how to find the network ID, broadcast ID, and valid host range for each subnet using both the binary method and a fast shortcut method across class A, B, and C examples.
What this lesson covers
- 00:25What subnetting is and why we use it
- 00:53Reading the subnet mask to detect subnetting
- 02:49Default subnet masks for classes A, B, and C
- 04:47Borrowing bits to create subnets
- 06:42Subnet mask values and the magic number
- 09:45Broadcast ID: next network ID minus one
- 10:05Class C /26 worked example
- 15:30Easy shortcut method: three examples
Key concepts
- Subnetting divides one large network into smaller networks, and every subnet gets its own network ID, broadcast ID, and range of valid host IPs.
- A network is subnetted when its subnet mask is longer than the default: more than /8 for class A, more than /16 for class B, and more than /24 for class C.
- Default subnet masks are /8 (255.0.0.0) for class A, /16 (255.255.0.0) for class B, and /24 (255.255.255.0) for class C, with class ranges 1 to 126, 128 to 191, and 192 to 223.
- Borrowing bits from the host portion creates subnets: borrowing n bits yields 2 to the power of n subnets (for example /24 to /26 borrows 2 bits for 4 subnets).
- The valid mask octet values are 128, 192, 224, 240, 248, 252, 254, and 255, built by adding the bit values 128, 64, 32, 16, 8, 4, 2, 1.
- Subnet size (block size) is found with the shortcut 256 minus the mask value, and subnets increment by that block size starting from zero.
- The broadcast ID of a subnet is the next network ID minus one, and the valid host range is network ID plus one through broadcast ID minus one.
- The network ID and the broadcast ID cannot be assigned to a host; only the addresses in between are usable IPs.
Key takeaways
- Always look at the subnet mask first, because it tells you whether a network is subnetted and lets you calculate the network ID, broadcast ID, and host range.
- Memorize the class ranges, the default masks, and the mask value chart, since fast subnetting depends on knowing these by heart.
- Use the magic number method (256 minus the mask value) to get the block size, then divide the host octet by that block size to locate which subnet an address belongs to.
- Practice many examples repeatedly across class A, B, and C, because subnetting speed and accuracy come with repetition.
Train for this the structured way
This free lesson is a sample of how CISNET teaches. The live courses cover it with guided labs and exam prep.
Frequently asked questions
How can I tell if an IP address is subnetted?
Look at the subnet mask. If the mask is longer than the default for that class, more than /8 for class A, more than /16 for class B, or more than /24 for class C, the network is subnetted. With only an IP address and no subnet mask, you cannot tell.
How many subnets do I get when I borrow bits?
The number of subnets is 2 to the power of the number of bits you borrow from the host portion. For example, moving a class C network from /24 to /26 borrows 2 bits and gives 4 subnets, while moving to /27 borrows 3 bits and gives 8 subnets.
How do I find the network ID, broadcast ID, and valid IPs of a subnet?
Find the block size with 256 minus the mask value, then list subnets starting at zero and incrementing by that block size. Each block start is a network ID, the broadcast ID is the next network ID minus one, and the valid host range runs from the network ID plus one to the broadcast ID minus one.
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