0.5 The tiny bit of background you need

Four ideas. Skim now; each is re-explained where it's used.

Idea 1 — Bits, bytes, and why 255 keeps appearing

A bit is a 0 or a 1. A byte is 8 bits.

With 8 bits you can count from 00000000 to 11111111:

  binary     decimal
  00000000  =   0
  00000001  =   1
  00000010  =   2
  ...
  11111111  = 255      <-- the biggest 8-bit number

That's why IP addresses are made of numbers 0 to 255, and why you'll see 255 constantly in subnet masks. There is nothing magic about it — it's just "all 8 bits switched on".

The place values of the 8 bits (memorise these; subnetting becomes easy):

   128   64   32   16    8    4    2    1
    |    |    |    |     |    |    |    |
    1    1    0    0     0    0    0    0   = 128 + 64 = 192
    1    1    1    1     1    1    1    1   = 255
    1    0    0    0     0    0    0    0   = 128

Idea 2 — Hexadecimal, in 30 seconds

Hex counts in 16s instead of 10s, using 0-9 then a-f:

  hex:      0 1 2 3 4 5 6 7 8 9  a  b  c  d  e  f
  decimal:  0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Two hex digits = exactly one byte (ff = 255). That's why hardware addresses look like a4:83:e7:2b:19:0c — six bytes, written as six pairs of hex digits. You rarely need to convert; you just need to not be scared of it.

Idea 3 — Client and server

• A server is a program that waits for connections and answers them.
• A client is a program that starts a connection and asks for something.

Your browser is a client. example.com's web server is a server. The same physical machine can run both. That's the entire distinction.

Idea 4 — A protocol is just an agreed format

A protocol is a set of rules two programs agree on: what the message looks like, what order things happen in, what each field means. Like agreeing that a letter has an address on the outside and a greeting at the top.

"TCP/IP" is a family of such agreements, which we'll meet one at a time.

That's genuinely all

No maths beyond counting. If you can read the place-value table in Idea 1, you can do everything in this guide, including subnetting.