Networking Basics


  OSI TCP/IP Stack
7 Application Application
6 Presentation
5 Session
4 Transport Transport
3 Network Internet
2 Data Link Link
1 Physical Physical

Protocol Data Units (PDUs)

Layer PDU
  1. Transport
  1. Internet
  1. Link
  1. Physical

Network topologies

Star topology “central switch”

  • if one link fails, the other links continue to function
  • central device is a SPOF
  • popular in modern networks

Mesh topology “multiple offices”

Allows for redundancy between each enpoint. In a full mesh, you need (n * (n - 1) / 2) links. Therefore, full mesh networks don’t scale well when the number of endpoints increases.

Links that are less useful can be removed. The network becomes a partial mesh. Endpoints can still reach each other by routing their traffic through other endpoints.

Full mesh Partial mesh
Optimal path Might be suboptimal path
Not Scalable More Scalable
More expensive Less expensive

Network architecture

Three Tiered

Layer Description
Access Layer Connected to endpoints
Distribution Layer Distributes the network to access layer
Core Layer Connect several distribution layers together (buildings, BUs etc…)

Collapsed Core

A collapsed core network is basically a three tiered network with the Distribution Layer and Core Layer merged into the so-called “Collapsed Core Layer”.

Cables and connectors


Nowadays we typically use Unshielded Twisted Pair cables with RJ45 connectors.

Copper cables
Name Type Notes
RG59 Coax lossy, for short distances only
RG6 Coax typical TV cable
RG58 Coax 50 Ohms, used for 10 Base 2
UTP Twisted pair unshielded twisted pair, typical cable today
STP Twisted pair shielded twisted pair, more expensive than UTP
Copper connectors
Name Type Notes
F Connector Coax  
BNC Coax Used with 10 Base 2
DB9 Serial Often used for management
RJ45/RJ11 Twisted pair Most common today
Twisted pair cable categories
Category Ethernet
Cat 3 10 Base T
Cat 5 100 Base TX
Cat 5e 1000 Base T
Cat 6 1000 Base T (thicker cables, better transmission)
Cat 6a 10G Base T
RJ45 wiring pinout. Pin 1 is left when holding the cable with the security clip down.
Pin T568A T568B
1 white/green white/orange
2 green orange
3 white/orange white/green
4 blue
5 white/blue
6 orange green
7 white/brown
8 brown

Fiber optic

  • Immune to electromagnetic interference
  • Can cover much larger distances than copper
  • LED or Laser (more powerful)
Fiber optic cables
Type Notes
single mode one ray of light, up to 40km. More expensive
multimode modal dispersion limits to ~2km
Fiber optic connectors
Type Notes
MTRJ 2 strands of fiber in one connector



Name Description
Top-down Start at the app level, eg. can I reach website from browser?
Bottom-up Start at the bottom, eg. is the cable plugged in?
Divide and Conquer Start in the middle and bisect, eg. start with ping


  1. Define the problem in a specific way
  2. Collect information
  3. Analyze information
  4. Eleminate unlikely causes
  5. Propose a hypothesis
  6. Test hypothesis (don’t interrupt people/systems if at all possible)
  7. Solve and document

If it didn’t work, start over at 2.