Stage 1 · Lesson 4 of 8

Subnetting fast, without a calculator

Subnetting means splitting a block of addresses into smaller networks. The quick method uses the : 256 minus the mask value in the octet that is being split. For a /26 the mask octet is 192, so the block size is 64 and start at .0, .64, .128 and .192.

The usable hosts are 2 to the power of the host bits, minus 2. Learn the table from /24 to /30 until you can recite it, then practise daily with random addresses. The exam gives you no calculator and time is tight, and at work you will check by eye in meetings.

Common trapstarting a subnet on a number that is not a multiple of its block size, such as a /27 at .20, which no will accept.

A question to check yourself, an AI practice session and 1 cheat sheet entry come with the full roadmap. Unlock

Resources for this stage

Start with· follow one of these

PlatformFree
Cisco Networking Academy
Cisco · Beginner
Start with the free, self-paced Networking Basics course; it uses Packet Tracer from the first modules
VideoFree
Computer Networking: A Top-Down Approach (Video Lectures)
Jim Kurose and Keith Ross · Intermediate
Free university-level lectures from the authors of the classic textbook, for the why behind every layer

Practise

Cisco Packet Tracer
Build a two-subnet network and use simulation mode to watch ARP, DHCP and pings step by step

Keep handy

Wireshark User's Guide
Capture your own traffic and read real headers; the display filter chapter is the one to bookmark

Also worth your time

TCP vs UDP: The Key Differences Every Developer Should Know (opens in a new tab)Read
A clear comparison of the two transport protocols and when each one fits
Beej's Guide to Network ProgrammingGo deeper
If you code, write a tiny TCP and UDP client and server to see the transport layer from the inside