OSI layers

  • Every network transmission is ruled by the OSI layers
  • Every protocol is in one of the seven layers
  • Every protocol is ruled by the IETF

OSI layers simplified

  • Layer 3 (L3): Internet layer (ex: IPv4, IPv6, ICMP, ICMPv6)
  • Layer 4 (L4): Transport layer (ex: TCP, UDP)
  • Layer 7 (L7): Application layer (ex: DNS, HTTP, SSH)

IP

  • Identifies a PC/server/VM/container on a network
  • Two type: IPv4 (55.212.24.127) IPv6 (fe80::f6b3:6105:cab3:89af)
  • Layer 2 protocol

IPv4

  • An IP comes with a network mask (netmask) to describe its network
  • All IPs inside the network are directly reachable

  • All IPs outside the network maybe reachable through a gateway

    henyxia@pdev:~ $ ip address
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
   valid_lft forever preferred_lft forever
2: ens18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
link/ether 02:8f:c5:23:e3:63 brd ff:ff:ff:ff:ff:ff
inet 10.2.1.12/16 brd 10.2.255.255 scope global ens18
   valid_lft forever preferred_lft forever

IPv4

  • An IP address can be displayed as integer or binary
    • 192.168.0.12 equals 11000000.10101000.00000000.00001100
  • Netmask can also be displayed as number of significant bit
    • 255.255.255.0 equals 11111111.11111111.11111111.00000000
    • 255.255.255.0 equals /24

IP and routes

  • Everything inside the network can be contacted directly
    • ex: 10.2.0.0/16 dev ens18 proto kernel scope link src 10.2.1.10

  • Everything outside the network can be contacted through a gatway

    • ex: default via 10.2.0.1 dev ens18 onlink

      henyxia@pdev:~ $ ip route
default via 10.2.0.1 dev ens18 onlink
10.2.0.0/16 dev ens18 proto kernel scope link src 10.2.1.10

Common networks

There are three kind of networks:

  • Reserved (almost usable by nobody)
  • Public (reachable by anyone, aka the Internet)
  • Private (usable only locally, not directly reachable from Internet)
    • 10.0.0.0/8
    • 172.16.0.0/12
    • 192.168.0.0/16

That’s all folks!

See you next week for the next episode!