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COSC 10001 · Week 7 · Wed, Oct 7, 2026

Networks &
the Internet

What happens when you hit Enter on a URL

Cold open

How do you turn on
a lamp?

Watch closely. We're going to check every word.

Howard's route · to a lamp fifteen feet away
geosynchronous orbit · 22,000 miles up💡1fibre to San Francisco2up to a satellite3down to Lisbon4undersea cable to Halifax5microwave relays6their ISP, and the lamp
The big idea

Nothing about the internet
is magic.

It's messages, addresses, and a lot of very fast copying.

1 of 3

Names aren't addresses

What you type, for humanstcu.edu
What computers route to54.162.35.76

DNS is the phone book in between. When a big site "goes down" for an afternoon, it is very often DNS.

2 of 3

Your data travels in pieces,
through strangers' machines

Hold that thought. That's next week.

So how does a packet know where to go?

It doesn't.

Nobody on the way knows the whole route. Every router knows one thing: which of its neighbours is closer to the address.

Like road signs. No sign knows the way from Fort Worth to Canberra. Each one just points "Australia: this way."

One packet, hop by hop
Pacific cable💻You · Fort Worth⇄Home router⇄Dallas⇄Chicago⇄Virginia⇄Los Angeles⇄Tokyo⇄Sydney🏛️Canberra · ANUto: 130.56.67.33not mine → Dallas130.56… → Los Angeles130.56… → Sydney130.56… → Canberrathat's me ✓

Look up the address. Pick a neighbour. Pass it on. Forget it.

Inside the Dallas router · simplified

A table of road signs

address starts withsend it to
130.56 · ANU, Australia→ Los Angeles
54.162 · Amazon, Virginia→ Chicago
1.1.1 · Cloudflare→ right here in Dallas
anything else→ upstream

A real one has about a million rows, and it picks one for every packet in a few billionths of a second.

Who writes the signs?
Pacific cable💻You · Fort Worth⇄Home router⇄Dallas⇄Chicago⇄Virginia⇄Los Angeles⇄Tokyo⇄Sydney🏛️Canberra · ANU130.56 is here!each router writes down: to reach 130.56, go → the neighbour I heard it from→ Canberra→ Sydney→ Sydney→ Los Angeles→ Los Angeles→ Dallas→ Chicago

Networks announce the addresses they can reach, and every router passes it on. It's called BGP.

Mon, Oct 4, 2021

Facebook told the internet,
by mistake, "you can't reach me."

Every router believed it. Facebook, Instagram, and WhatsApp vanished for about six hours. One bad announcement, and a company the size of a country fell off the map.

3 of 3

Distance is time

Money can't make light faster.
Hold on to that sentence.

The demo

Let's check
Howard's route.

Three free commands, already on your laptop.

Check 1 · where is TCU's website?
nslookup www.tcu.edu
The answer ends in

us-east-1.
amazonaws.com

TCU's website doesn't live at TCU. It lives in an Amazon data center in Northern Virginia, about 1,200 miles away.

Check 2 · how long is the round trip?
ping -c 4 www.tcu.edu

On Windows, leave off the -c 4.

Request timeout

Is TCU's website down?

No. Amazon just doesn't answer pings.

A failed command tells you something too.
Just not what you'd guess.

Check 3 · near versus far
ping -c 4 1.1.1.1
ping -c 4 www.anu.edu.au

A server a few miles away, then the Australian National University in Canberra.

How traceroute sees the hops
Pacific cable💻You · Fort Worth⇄Home router⇄Dallas⇄Chicago⇄Virginia⇄Los Angeles⇄Tokyo⇄Sydney🏛️Canberra · ANU$ traceroute www.anu.edu.aulimit 1💥too far! I'm home router1 2 ms home routerlimit 2💥too far! I'm Dallas2 7 ms Dallaslimit 3💥too far! I'm Los Angeles3 37 ms Los Angeleslimit 4💥too far! I'm Sydney4 170 ms Sydneylimit 5here! I'm Canberra5 188 ms Canberra ✓

Every packet carries a hop limit. Each router takes one off. Whoever hits zero throws it away and says so, and that reply gives away where it is.

Check 4 · every hop on the way
traceroute -q 1 -w 1 www.anu.edu.au

Windows: tracert www.anu.edu.au. The hop names say where they are.

What we just saw
Dallas~7 ms
Los Angeles~37 ms
Sydney · the bottom of the Pacific~170 ms
Canberra~188 ms

LA to Sydney is about 12,000 km. At 200,000 km per second that's 60 ms there and 60 back: 120 ms.

We just measured the speed of light
from a classroom in Fort Worth.

Check 5 · another Australian university
ping -c 4 www.unimelb.edu.au
Faster than Los Angeles

Did Melbourne move?

nslookup www.unimelb.edu.au
A content delivery network

A copy of the website,
parked near you

Money can't make light faster.
So you spend it moving the data closer.

That's the "very fast copying." It's most of why the internet feels instant.

One more · Chrome, DevTools, Network tab

You asked for one page.

Count the requests at the bottom. Each one is a little trip like the ones we just traced.

Howard, checked
geosynchronous orbit · 22,000 miles up💡✅ fibre: real❌ satellite: 240 ms, for a lamp✅ sea-floor cables: real🤷 relays exist, not like that

Our trace to the far side of the planet took 190 ms. Howard's satellite alone costs more.

So how did Howard do it?

The internet would never
choose his route.

Every router picks the shorter way. To go through Lisbon, he had to force it: send the message to a computer in Lisbon, which sends it on to one in Halifax, and so on.

Possible, and pointless.
"Because we can."

A fluent, confident explanation
was partly wrong.
A thirty-second command settled it.

Your agent talks exactly like Howard.

So what

Somebody chose to put TCU's website in Virginia.
That's a job.

Before you go
Build Checkpoint 1. Your Karel repo, a world you built, the agent's commit, NOTES.md. Push, and make sure the repo is public. Submit the URL as a text submission in TCU Online by 11:59 PM.today
Cybersecurity. Strangers' machines, and what they can readWed, Oct 14
Peer Mentor Meeting. Book it this weekWed, Oct 21
CS Engagement, 2 of 3Wed, Oct 21

I grade Checkpoint 1 by cloning your repo and running it. If it doesn't run on my laptop, I can't see it.

Messages, addresses,
and a lot of very fast copying.

And when something sounds sure of itself: check it.