Lesson 1 of 7

The Big Picture: Packets & Layers

The internet splits every message into small packets, sends each one separately and reassembles them at the other end. This lesson explains packets and the layers that carry them.

There is no wire between you and a website. Your is split into packets (~1,500 each), each stamped with source and destination addresses. Routers — machines whose whole job is 'read the address, forward toward it' — pass each packet hop by hop. Packets may take different routes, arrive out of order, or vanish; the sort it out. This design copes well with failures: if a router goes down, packets route around it.

Text

The layers (each one only talks to its neighbors):

  Application   HTTP, DNS, SSH — what the data MEANS
  Transport     TCP/UDP — reliable delivery, ports
  Internet      IP — addressing & routing between networks
  Link          Ethernet, WiFi — the next physical hop

A packet is envelopes inside envelopes:
  [Ethernet [IP [TCP [HTTP "GET / ..."]]]]

Each hop unwraps the outer envelope, reads the IP
address, and re-wraps for the next physical link.

Bash

# Watch your packets cross the world:
traceroute example.com
#  1  192.168.1.1      1ms   your router
#  2  10.20.0.1        8ms   your ISP
#  5  core2.chi.net   25ms   a city backbone
#  9  93.184.216.34   85ms   the destination

# Each line = one router hop. Latency jumps reveal
# oceans: ~70ms extra = an Atlantic crossing.
# (Physics floor: light in fiber does ~200km per ms.)

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