UNIX Internet

Last modified Monday, 24-Apr-2023 16:43:27 UTC.

The Internet
- Packet Switched
  - Data is transmitted in packets
  - Packet = data + destination information (e.g. IP address)
  - The packet is sent from one machine to the next until the destination machine is reached
  - Routers coordinate the network load and transmissions of packets
- IP numbers
  - Each machine has a 32 bit IP number, e.g., 137.219.47.69
    - IP number has 4 octets, in two parts: network and machine
    - 1st octet 1-127
      - Class A network
      - Network number in 1st octet, machine number in others
      - 127 networks
      - 16M machines
    - 1st octet 128-191
      - Class B network
      - Network number in 1st and 2nd octet, machine number in others
      - ~16K networks
      - 64K machines
    - 1st octet 192-223
      - Class C network
      - Network number in 1st to 3rd octet, machine number in last
      - ~2M networks
      - 255 machines
    - 1st octet 224 onward
      - Multicast network
  - Network Address Translation (NAT)
    - A way to map multiple IP addresses inside a local network to a single IP address outside the network (the public internet).
- Logical
  - Hierarchy of domains
  - Names are resolved to IP numbers by DNS computers
  - Routers and gateways forward packets to the required IP address
- Protocols
  - Network level
    - IP - Smallest data level. Unreliable and unordered
    - RIP - For sharing routing data
    - ARP - For resolving names to addresses
  - Transport level
    - UDP - On IP to make it reliable, with service multiplexing via ports concept
    - TCP - On IP to make it connection based, reliable, and stream-oriented (ordered packets)
- Port numbers
  - Identifies a process to which an internet message is to be forwarded at a server.
  - Well-known port numbers 0-1023 are reserved for known usages. See /etc/services for known ports up to 1024, e.g., ...
    - 21 = File Transfer Protocol (FTP)
    - 22 = Secure Shell
    - 25 = Simple Mail Transfer Protocol (SMTP)
    - 53 = Domain Name System (DNS)
    - 80 = HTTP
    - 443 = HTTPS
  - Registered port numbers 1024-49151 used by companies
  - Dynamic port numbers 49152-65536 available for anyone to use
Some useful commands
- ping machine to check if machine is reachable
- nslookup to get information about a machine on the net.
  - Provide a computer name.domain to get the IP address
  - Provide an IP address to get the computer name.domain
  - set debug to see lots of details
  - So much more
- dig is a newer version of nslookup
- traceroute machine to print the route that packets take to machine
- netstat symbolically displays the contents of various network related data structures
  - Plain netstat tells about all internet connections
  - netstat -s gives a summary of network activity
- finger login or finger login@machine shows details (well, mostly turned off these days) for the login
- ssh machine command securely runs command on machine where you have an account. If the command is omitted you get a login prompt.
- scp machine1:filename1 machine2:filename2 to copy files between machines

UNIX Internet

Exercises

Exam Style Questions