Addresses, Masks and Subnets

The arithmetic every other decision in this course depends on

Step 1 of 2 — understand the idea. Read this first and the packet trace will confirm what you already expect, instead of teaching you two things at once.

What you'll be able to do

▸Split an address into its network and host portions using a prefix length
▸Decide whether two addresses are on the same subnet, by calculation
▸Name the address ranges that are reserved and say what for

Before this lesson — it builds directly on:

A MAC address identifies a device and says nothing about where it is. That is fine on one wire and useless on a network of networks — a router receiving a packet for a MAC address it has never heard of would have nowhere to send it. An IP address is structured. Part of it says which network, and the rest says which machine on it. That structure is what makes routing possible: a router does not need to know about every machine in the world, only about every network, and there are far fewer of those. The mask is what says where the split falls, and every forwarding decision on the internet begins with that split.

Watch these fields in the lab: ipv4.destIp · ipv4.sourceIp

Prefix length: How many leading bits of the address are the network part, written after a slash: /24 means the first 24 bits.
Subnet maskglossary: The same value written as an address: ones covering the network bits, zeros over the host bits. /24 is 255.255.255.0.
Network address: The address with every host bit zero. Names the subnet itself and is never given to a machine.
Broadcast addressglossary: The address with every host bit one. Reaches every machine on the subnet, and is never given to a machine either.

✓ Concept check — before you open the packets

These test the idea, not the trace. You should be able to answer them from the explanation above.

Which mask does a host use to decide whether a destination is local?

Now that you understand the concept, observe how it appears in the packet exchange.

The lab runs a real simulated capture — pause, step, click any packet, and inspect every byte.