Foundations Of Networking
Network Types and Topologies
from carriers; latency is tens to hundreds
JrCodex·7 min read
Jr Codex Computer Networks Notes
Level: Beginner Prerequisites: Chapter 3: Encapsulation Time to complete: ~15 minutes
Table of Contents
- Networks by Scale
- Topologies
- Circuit vs Packet Switching
- Client-Server and Peer-to-Peer
- The Internet Is a Network of Networks
- Summary & Next Steps
1. Networks by Scale
The Categories
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PAN Personal Area Network
metres. Bluetooth, USB, a phone and its
earbuds.
LAN Local Area Network
a building or campus. Ethernet and wifi.
You own the equipment; latency is
sub-millisecond.
MAN Metropolitan Area Network
a city. A university's campuses, a cable
operator's coverage.
WAN Wide Area Network
countries and continents. You lease it
from carriers; latency is tens to hundreds
of milliseconds.
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The Distinction That Actually Matters
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Not the size. Whether YOU CONTROL IT.
ON YOUR LAN
predictable latency, negligible loss, known
equipment, and you can change it.
ACROSS A WAN
variable latency, real packet loss, equipment
owned by others, routes that change without
notice.
Almost every design decision in this curriculum
is different on one side of that line than the
other — and the classic distributed-systems
mistake is treating a WAN like a LAN.
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2. Topologies
The Shapes
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BUS all devices on one shared cable
A──B──C──D
+ cheap
- one break kills everything; only one
device may transmit at a time
Obsolete, but its COLLISION problem
shaped Ethernet (Module 2, Chapter 3)
RING each device connects to two neighbours
+ predictable, no collisions
- one break splits the ring
Survives in some fibre backbones
STAR everything connects to a central switch
B C
\ /
A───S───D
+ one failure affects one device;
easy to add devices
- the switch is a single point of
failure
── what essentially every LAN is today
MESH many devices interconnected
+ highly resilient; multiple paths
- expensive; n(n-1)/2 links for full
mesh
── the shape of the internet's core
TREE hierarchical stars
── how campus and datacentre networks
scale
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Physical vs Logical
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These can differ, and often do.
Wifi is PHYSICALLY a star — every device talks to
the access point.
It is LOGICALLY a bus — everyone shares one radio
medium, so only one may transmit at a time, and
collisions are possible.
That mismatch is exactly why wifi needs a
different access method than switched Ethernet
(Module 2, Chapter 3).
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3. Circuit vs Packet Switching
CIRCUIT SWITCHING
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Reserve a dedicated path for the whole
conversation before any data flows.
The traditional telephone network.
+ guaranteed bandwidth and constant latency
+ no per-message addressing overhead
- the path is reserved even while SILENT — a
phone call with nobody talking still holds the
full circuit
- setup delay before anything can be sent
- a failed link kills the call
PACKET SWITCHING
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Split data into packets. Each is addressed and
routed independently. Links are shared.
The internet.
+ STATISTICAL MULTIPLEXING — an idle
conversation consumes nothing, so far more
users fit on the same capacity
+ resilient — packets route around failures
+ no setup delay
- variable latency (JITTER), and queues
- packets may be lost, duplicated or reordered
- per-packet header overhead (Chapter 3)
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Why Packet Switching Won
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Data traffic is BURSTY. You load a page, then
read it for thirty seconds, then load another.
Circuit switching would hold the full capacity
through all that reading. Packet switching lets
a thousand people share a link that could carry
only ten simultaneous circuits.
The cost — variable latency and possible loss —
is exactly what the transport layer was invented
to hide (Module 4).
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4. Client-Server and Peer-to-Peer
CLIENT-SERVER
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Clients request; a server responds. Asymmetric.
+ central control, easy to secure and update
+ one authoritative copy of the data
- the server is a bottleneck and a single point
of failure
- capacity must be provisioned for peak
The web, email, databases, APIs.
PEER-TO-PEER
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Every node is both client and server.
+ capacity GROWS with the number of users — more
peers means more upload bandwidth
+ no single point of failure
- hard to secure, hard to find who has what
- unpredictable availability; peers leave
BitTorrent, blockchains, some video calling.
The Property Worth Understanding
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In client-server, demand scales with users and
supply does not. A thousand users downloading a
file need a thousand times the server bandwidth.
In peer-to-peer, each new user brings their own
upload capacity. Demand and supply grow together.
This is why P2P distribution of large files is
fundamentally cheaper — and why CDNs (Module 7,
Chapter 2) exist to give client-server systems
some of the same property.
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The Common Hybrid
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Most real systems are both:
- a central server for DISCOVERY,
authentication and coordination
- direct peer connections for the BULK DATA
Video calling works this way: a server matches
the participants, then media flows peer-to-peer
where possible.
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5. The Internet Is a Network of Networks
The Structure
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The internet is not one network. It is ~75,000
independently operated networks that agree to
carry each other's traffic.
AUTONOMOUS SYSTEM (AS)
One network under one administrative control,
with its own routing policy. Your ISP is an AS.
So is a large university, and so is Google.
Each has an AS number.
TIER 1 global backbones that reach everywhere
without paying anyone. They PEER with
each other.
TIER 2 regional; peer where they can, pay
Tier 1 for the rest
TIER 3 local ISPs; mostly buy transit
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PEERING vs TRANSIT
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TRANSIT you PAY a provider to carry your
traffic to the whole internet.
PEERING two networks exchange traffic
DIRECTLY, usually settlement-free,
because it benefits both.
This is why the internet's routing is shaped by
COMMERCIAL relationships as much as by geography
— and why the shortest physical path is often
not the path your packets take (Module 3,
Chapter 4).
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Why It Matters to You
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It explains things that otherwise look like bugs:
- a packet from London to Paris routing via
Amsterdam, because that is where the two
networks peer
- latency to a nearby server being worse than to
a distant one
- an outage at a network you have never heard of
breaking your service
No single organisation controls the path. Module
3 covers how routing decisions are actually made
across these boundaries.
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6. Summary & Next Steps
Key Takeaways
- The meaningful distinction is not size but control: LAN latency is predictable and loss is negligible, while across a WAN neither is true.
- Physical and logical topology can differ — wifi is a physical star and a logical bus, which is why it needs a different medium access method.
- Packet switching won because data is bursty, and statistical multiplexing lets far more users share a link; the price is variable latency and loss.
- The internet is roughly 75,000 independently run networks whose routing is shaped by commercial peering and transit relationships, not by geography.
Concept Check
- Why is "do you control it" a more useful distinction than LAN versus WAN by size?
- Explain why peer-to-peer distribution of a large file scales differently from client-server.
- Why might packets from London to Paris travel via Amsterdam?
Next Chapter
→ Chapter 5: Measuring a Network
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