Member Avatar for lewashby

I was going fair on my Udemy networking course until I go to lesson 47. Subnetting Large Networks Part 2, my brain started smoking. Up to this point I had been dividing the network/hosts on the forth octet. And when counting the bits that I was going to borrow from the host portion of the address I wouldn't even count the bits in the network portion of the address. If it was class C I would only count the bits in the forth octet, if it was a class B I would only count the bits from bit 17 and forward/right, and if it was class A from 9 forward/right. But when I got to this lesson the instructor divides the network on the third octet and for whatever reason he started counting the network bits from the beginning, the very first bit in the first octet. I've attached the file. My network/cicso training has come to a dead stop.

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Treat the IPv4 address as a 32‑bit binary string and think in terms of a prefix length. The network portion is always the leftmost N bits and the host portion is the remainder. Subnetting means turning some of the leftmost host bits into extra network bits — i.e., borrow from the host side starting at the first host bit (the most significant host bit). That is why an instructor drawing a full 32‑bit map can look like they are “counting from the very first bit” even though the borrow actually begins immediately to the right of the current network boundary. The attachment mentioned by is not present, as noted.

A compact, repeatable method works in every case:

  1. Establish the current prefix (classful default or given).
  2. Decide how many subnets (or hosts) are needed and find the smallest b with 2^b >= needed subnets.
  3. New prefix = base prefix + b. Convert that to dotted mask or binary to see which octet contains the last borrowed bit. The subnet increment occurs in that octet.
    Examples commonly used: from /24 borrow 2 → /26 → mask 255.255.255.192, increment 64 in the fourth octet (.0, .64, .128, .192). From /16 borrow 4 → /20 → mask 255.255.240.0, increment 16 in the third octet.

Quick troubleshooting tips: draw a 32‑box grid, mark the base network bits, mark the borrowed bits, then convert the changed octet to decimal to get increments. Note older texts warn about all‑zero/all‑ones subnets; most modern gear accepts them, but check lab/exam rules.

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Member Avatar for Neeraja_2

The total number of network bits are calculated from the first bit of the first octet. In class C addressing since the first three octets are for the network, we usually count the number of additional bits from the fourth octet and add it to 24.

Member Avatar for Dani

You say you’ve attached the file but I’m not seeing a post attachment?

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