What is a Subnet Mask??? (you NEED to know it!!)

About this video

This CCNA lesson explains why a subnet mask works by converting it to binary alongside an IP address. Using the place-value chart from the previous episode (128, 64, 32, 16, 8, 4, 2, 1), 255 becomes eight 1 bits and 0 becomes eight 0 bits. In the mask, 1 bits mark the network portion of the address that does not change, which explains the earlier shortcut that a 255 freezes the corresponding octet, and 0 bits mark host bits that can vary per device. Counting host bits gives network size: usable hosts equal 2 raised to the number of zero bits, minus 2 for the network/subnet address and the broadcast address. For a 255.255.255.0 mask that is 2 to the 8th equals 256 addresses, or 254 usable. Subnetting is then introduced as borrowing a network bit to become a host bit: with nine zeros the network holds 512 addresses, 510 usable, and converting that altered third octet back to decimal changes the mask to 255.255.254.0. He stresses mask bits are contiguous — a run of 1s followed by a run of 0s — and defers address ranges to the next episode.

Key takeaways

• Convert the mask to binary: 255 is all 1s and 0 is all 0s because the octet place values sum to 255.
• Mask 1s identify network bits that stay fixed; mask 0s identify host bits that can be assigned to devices.
• Usable hosts formula: 2^(number of zero bits) minus 2, accounting for the network and broadcast addresses.
• A mask of 255.255.255.0 yields 256 total and 254 usable addresses; needing more hosts requires more zero bits.
• Subnetting is changing the mask by borrowing a network bit: one borrowed bit in the example doubles capacity to 510 usable and makes the mask 255.255.254.0.

Episode details: published July 19, 2022 · 12:59 long. Summary written by Self Growth Videos from the episode itself. Business results and any income figures mentioned are NetworkChuck’s own claims, shared for education — they are not typical or guaranteed.

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