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TechKnowSurge Catalog Videos DEMO: Default Subnet Masks
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DEMO: Default Subnet Masks

Default subnet masks define the network and host portions of an IP address for each address class. Understanding how to identify them is a foundational skill for subnetting.

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About this video

Default subnet masks are tied to the classful addressing system and define how an IP address is divided between its network and host portions. Each address class carries a standard default mask: Class A uses 255.0.0.0, Class B uses 255.255.0.0, and Class C uses 255.255.255.0. These masks reflect how many octets are reserved for network identification within each class. Although classful addressing has largely been superseded by classless inter-domain routing (CIDR), a solid understanding of default subnet masks remains important for building subnetting proficiency. Correctly identifying the class of an IP address and applying the appropriate default mask is a foundational skill that underpins more advanced network segmentation concepts. Mastery of this baseline prepares network professionals to work confidently with variable-length subnet masking and modern IP address planning.

What you'll learn

What's covered

Default Subnet Masks

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

IP Address
A numerical label assigned to each device connected to a network that uses the Internet Protocol.
Subnet Mask
A 32-bit number that divides an IP address into network and host portions.
Subnet
A logical subdivision of an IP network, used to organize and segment network traffic.
IPv4 Address Class
One of five categories (A–E) into which IPv4 addresses are grouped based on leading bit patterns, each defining a specific range of addresses and a fixed network-to-host bit boundary.

Transcript

Although masks really weren't needed when we had classful addressing and just classful addressing, by understanding the default mask we can better subnet. So let's identify default subnet masks for different networks, or different addresses.

Here I have a bunch of addresses. What we're going to do is take a look at what those addresses are and determine the class. In this case right here it's a Class B, and so then determine what the default subnet mask would be for Class B. A default subnet mask for a Class B would be 255.255.0.0, because the first two octets are the ones that are the network portion. These are the network portion right here, so it would be 255.255.0.0.

Go through all of these and figure out what the answers are, then check your work against the answers.

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