I am pursuing a CCNA (Cisco Certified Network Associate) course and I decided to write my notes in markdown. This website also serves as a great way to easily share my work. These are all my personal writings, so if you find any mistake, don’t hesitate to contact me so I can fix it!

1. Introduction

Types of networks, by model:

  • Client - Server: the client requests content which is processed and sent by the server
  • Peer to peer: each device can be either a client or a server, no dedicated server is required

Types of networks, by size:

  • LAN = Local Area Network (home, school, company size)
  • MAN = Metropolitan Area Network (multiple LANs)
  • WAN = Wide Area Network (biggest one, multiple MANs)
  • WLAN = Wireless LAN
  • SAN = Storage Area Network
  • PAN = Personal Area Network (Bluetooth)

Types of networks, by destination:

  • Home (cable, cellular network, satellite, dial-up)
  • Business (metro ethernet, business DSL, satellite)

Business networks:

  • intranet: accessible only inside the business
  • extranet: resources accessible to the outside users
  • internet: both

Properties of a network:

  • fault tolerance
  • scalability
  • quality of service: prioritize traffic based on its type (example: voice > video)
  • security

Type of network attacks:

  • viruses, worms, and trojan
  • spyware, adware
  • zero-day attacks
  • denial of service

Networking trends:

  • BYOD = bring your own device
  • online coworking
  • cloud computing
  • powerline networking
  • IoT = Internet of Things
  • SDN = Software-defined Networking

Key questions:

  • What’s a network?
  • How to represent a network?

2. Communication Protocols

Protocol = a set of rules (such as language, message format, message dimension, transmission speed)

A message protocol may contain the address source, destination source and the rest of the data.

Delivery options:

  • Unicast: communication one-to-one (1 source, 1 destination)
  • Multicast: 1 source, multiple destinations
  • Broadcast: 1 source, everyone else in my network is listening

Protocol stacks: separation of concerns

The TCP/IP stack (simplified):

  • Application: user interaction
  • Transport: data communication between devices
  • Network: find the best route between two devices
  • Environment Access: hardware communication

The OSI stack:

  • Application: user interaction
  • Presentation: uniform data representation
  • Session: keep communication alive between two devices
  • Transport: segment data and send packets
  • Network: find the best route between two devices
  • Data link: communication between devices within the same environment
  • Physical: from bits to hardware communication

Standard = document that provides information on how a protocol is defined

RFC = Request for Comments (developed by IETF)

IP = Internet protocol

ISO = International Organization for Standardization

IEEE = Institute of Electrical and Electronics Engineers

Transmission methods:

  • segmentation: gets split into ordered packets such as 1, 2, 3, 4, …
  • multiplexing: merge different sources in order such as 1, 1, 2, 2, …

Key questions:

  • How do multiple devices communicate?
  • What is a communication protocol?

3. IOS Configuration

IOS = Internetwork Operating System, Cisco’s operating system which runs on their devices

Components of an operating system:

  • shell: interface which allows the user to interact with the OS
  • kernel: enables communication between the hardware and the OS
  • hardware: the physical components of a device

Two ways to interact with an OS:

  • GUI = Graphics User Interface
  • CLI = Command Line Interface

Fun fact: Apple negotiated with Cisco the rights for the name “IOS” since Cisco owns the trademark: link.

IOS Features:

  • security
  • quality of service (QoS): prioritize traffic based on its type (example: voice > image)
  • routing: find the best route for a packet
  • resources: memory, battery, etc.
  • addressing: configure different addresses for the network
  • interfaces: read port values

CLI Access:

  • Direct:
    • console port
    • AUX port: deprecated
  • Remote:
    • Telnet
    • SSH: secure shell, data is encrypted (which makes it better than telnet)

IOS hierarchy:

IOS has multiple modes, for various configuration use cases

Modes:

  • User EXEC (>): default mode
  • Privileged EXEC (#): enabled with the command “enable”
    • Global config: “configure terminal”
      • Interface: “interface interf-name interf-number”, example names: fa0/1, gig0/0
      • Line: “interface line-name line-number”
      • Router: “router routing-protocol-name”

Other commands:

  • exit to go back one mode
  • end to go back to the root mode

Common commands:

  • How to change the hostname:

    plaintext
    Router> enable
Router# configure terminal
Router(config)# hostname Constanta
Constanta(config)# exit
Constanta#

    The hostname must contain only alphanumeric characters and/or ”-”.

  • How to change the password:

    Unencrypted:

    plaintext
    Constanta> enable
Constanta# configure terminal
Constanta(config)# enable password cisco
Constanta(config)#

    Encrypted:

    plaintext
    Constanta> enable
Constanta# configure terminal
Constanta(config)# enable secret cisco
Constanta(config)#

    On a direct connection:

    plaintext
    Constanta> enable
Constanta# configure terminal
Constanta(config)# line console 0
Constanta(config-line)# password cisco
Constanta(config-line)# login

    On a remote connection:

    plaintext
    Constanta> enable
Constanta# configure terminal
Constanta(config)# line vty 0 15
Constanta(config-line)# password cisco
Constanta(config-line)# login

PDU = packet data unit

4. Physical layer

The physical layer, the first one of the OSI stack, works with bits.

The PDUs of the OSI stack layers:

HardwareLayerPDU
HubPhysicalBits
SwitchData LinkFrames
RouterNetworkPacket
TransportSegment
SessionSPDU
PresentationPPDU
End DevicesApplicationAPDU

Physical Layer properties:

  • transforms the received frames in bits
  • sends the data as signals
  • receives signals from the environment, sends them back as bits
  • sends data to layer 2

How it converts the bits as signals:

  • copper: Electricity
  • optical fiber: light
  • wireless: radio

Network devices:

  • NIC = Network Interface Card, connects the equipment to the network
  • Interface = the port
  • Cable = connects the devices
  • Connector = the end of a cable

Specific terms:

  • Bandwidth:
    • the maximum theoretical speed of a network
    • bps = bits per second
    • we can limit the bandwidth of a specific app
  • Throughput:
    • the actual number of transmitted bits
    • always smaller than the bandwidth
    • bps
  • Latency:
    • the time it takes for the data to reach the destination
    • seconds
  • Goodput:
    • number of useful bits transmitted in a period of time
    • smaller than throughput

Copper cables:

  • 2 types:
    • UTP = unshielded twisted pair
    • STP = shielded twisted pair
  • connectors:
    • RJ-45
  • cabling standards:
    • T568A: from the USA
    • T568A: from the EU
  • interferences:
    • EMI + RFI
    • crosstalk

Cabling standard rules:

  • full color + color with white
  • always brown cables at the end
  • always blue in the middle
  • T568A:
    • 12345678
      *greengreen*orangeblue*blueorange*brownbrown
  • T568B:
    • 12345678
      *orangeorange*greenblue*bluegreen*brownbrown
    • before a color means it’s combined with white

Cabling types:

  • straight-through cables:
    • both ends use the same standard
    • between different equipments
    • host <-> switch
    • switch <-> router
  • crossover cables:
    • between equipments of the same type

Fiber optic:

  • types:

    • single mode (SMF)
    • multi mode (MMF)

  • structure:

    • core
    • coating
    • outer jacket

  • connectors:

    • straight-tip (ST)
    • subscriber connector (SC)
    • lucent connector (LC)

  • can’t be affected by interferences

  • testing can be done with an OTDR

Fiber vs. Copper:

CrietiriaCopperFiber
bandwidth10Mbps-10Gbps10Mbps-100Gbps
distancesmall(1m-100m)large(1m-100000m)
interferencesEMInope
costlowhigh
instalationeasyhard

Wireless:

  • types: WiFi, Bluetooth, WiMax, Zigbee
  • WLAN: IEEE 802.11
  • Ethernet: IEEE 802.3
  • WPAN (bluetooth, rfid): IEEE 802.15

Bit encodings:

  • NRZ = Non-Return to Zero, 1 when a certain voltage is exceeded, otherwise 0
  • Manchester = map a voltage range (-v to +v) from 0 to 1

Responsibilities:

  • controlls the enviorment access (wireless, copper or optic cables)
  • detects errors
  • recieves data from layer 3
  • works with frames

Sublayers:

  • LLC = Logical Link Control (software, communicates with layer 3); IEEE 802.2
  • MAC = Media Access Control (hardware, works with layer 1); 802.3, 802.11, 802.15

LAN Topologies:

  • LAN = local area network
  • star, bus, ring

WAN Topologies:

  • WAN = wide area network
  • Point to point, hub and spoke, mesh

Enviorment types:

  • Half duplex (one way communication)
  • Full duplex (two channels, for communication and receiving)

Wireless is half duplex.

Ethernet

MAC Address: switch

IP Address: router

Mac Address:

  • burned in
  • 48 bits
  • unique
  • examples: OUI (organizational unique id) + VA (vendor assigned)

Types of MAC addresses:

  • Broadcast (FF:FF:FF:FF:FF:FF)
  • Multicast (01:00:5E:XX:XX:XX)
  • Unicast

Ethername frame:

  • header
  • destination address
  • source address
  • type
  • data
  • FCS

Address Resolution Protocol (ARP):

  • IP to MAC mappings
  • timer
  • command: “shop ip arp”

Switches:

  • don’t have an ARP table (for now :D)
  • routers use ARP, switches use CAM
  • they use the CAM table
  • CAM = Content Addressable Memory, aka the brain of a switch
  • the CAM table has MACs to interfaces mappings
  • “show mac-address-table”

CAM Algorithm:

  • source MAC: is the MAC in the table?
    • yes: resets the timer to 300
    • no: add the new MAC address with the coresponding port (e.g. Fa0/1)
  • destination MAC: is the MAC in the table?
    • yes: send the frame on the coresponding port
    • no: flooding (sends back on all interfaces except the source one)

CAM table example:

MAC AddressInterfaceTimer
AF:A1:10:11:FF:BAFa0/1300

Buffering methods:

  • port based: each port has its own queue
  • shared memory: ports share the same memory space

Collision domains:

  • extended by: Hub
  • delimited by: Router, Switch, End Devices

Number of broadcast domains = number of networks

6. Network layer

IPv4 & IPv6

IPv4IPv6
Number of bits32128
Number of fields128
Required processingTTL modification & HCS recalculationmodify the Hop limit

How routing works:

  • is the destination localhost (127.0.0.1 or ::1)?
  • is the destination in the same network?
  • is the destination in another network? send the packet to the Default Gateway

Default Gateway:

  • an IP adress
  • the router’s interface that I am connected to

Routing Table:

  • the router’s brain
  • routes from the same network are automatically added
  • routes outside the local network can either be static (added by the network admin) or dynamic (discovered using a routing protocol)

IP Addressing

IPv4:

  • 32 bits
  • network & host parts
  • network identifies its network (LAN)
  • host identifies the device

Mask:

  • one blocks of 1s then a portion with only 0s
  • example: 255.255.240.0 => last 12 bits are for the host
  • short notation: “/20” => first 20 bits are 1
  • 1s for the network and 0s for the host

Types of addresses:

  • network address:

    • can’t be assigned to an equipment

  • broadcast address:

    • can’t be assigned to an equipment

  • allocable address:

    • can be allocated to an equipment

Example:

  • 129.100.68.224 /21
  • 1000 0001 0110 0100 0100 0100 1110 0000 - ip
  • 1111 1111 1111 1111 1111 1000 0000 0000 - mask
  • 1000 0001 0110 0100 0100 0100 0000 0000 - ip & mask = network address (129.100.64.0 /21):
  • 1000 0001 0110 0100 0100 0111 1111 1111 - ip | ~mask = broadcast address (192.100.72.255 /21):
  • which means:
    • from 129.100.64.0/21 to 129.100.71.255/21 are assignable IPs in my network

Classful addressing:

  • A: /8
  • B: /16
  • C: /24
  • D: N/A
  • E: N/A

Private addresses used only in private networks:

  • 10.0.0.0 - 10.255.255.255 => /8
  • 172.16.0.0 - 172.31.255.255 => /12
  • 192.168.0.0 - 192.169.255.255 -> /16

ICMP

  • the protocol used in the ping utility
  • sends a message between two devices
  • checks errors than can occur in communication

Types of messages:

  • RA = router advertisment (sent every 200s)
  • RS = requests the IPv6 address of all routers
  • NS = checks if an IPv6 address is unique
  • NA = answer to NS (neighbour advertisment)

Transport layer

  • works with segments or datagrams

  • virtual ports

  • splits data into smaller packages

  • reassembles the data on its destination

  • well known ports: 0-1023

  • registered: 1024-45151

  • private: 45152-65535

portprotocolapplication
20TCPFTP Data
21TCPFTP Control
22TCPSSH
23TCPTelnet
25TCPSMTP
53TCP/UDPDNS
67,68UDPDHCP
69UDPTFTP
80TCPHTTP
110TCPPOP3
443TCPSSL/HTTPS
  • TCP = transmission control protocol (reliable, safe)
  • UDP = user Datagram protocol (unreliable, fast)

TCP

  • Header:

    • source & destination ports (each 16 bits)
    • sequence number = SEQ
    • acknoledgement number = ACK
    • header length
    • reserved: 6 bits of zero
    • controk bits: URG, ACK, PSH, RST, SYN, FIN
    • window: hints about how much data the current stream can handle
    • checksum
    • urgent
    • options

  • Creating a TCP connection (3 way handshake):

    • Header view:
      • -> Seq = 69, ACK = 0
      • <- Seq = 42, ACK = 69 + 1
      • -> Seq = 70, ACK = 42 + 1
      • <- Seq = 43, ACK = 70 + 1
    • Control Bits view:
      • -> SYN
      • <- SYN + ACK
      • -> ACK

  • Closing a TCP connection (also a 3 way handshake):

    • -> FIN
    • <- ACK
    • <- FIN
    • -> ACK

UDP

  • fast but not safe

  • doesn’t resend data

  • smaller header

  • doesn’t confirm events

  • Header:

    • source & destination ports
    • length
    • checksum