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special ipv4-adresses

  1. localhost, localnet
     
    To test the IP stack without using a physical network card, we make use of localhost.
    localhost has the IP address 127.0.0.1.
    • you can ping your 127.0.0.1
    • you can test services on your server using this address, e.g.: ssh -p 22 user@localhost
    • if you are running a website, you can test it by using 127.0.0.1 as the URL in your browser
       
      With netstat, you can see which connections are open on your machine, and these very often refer to localhost or its IP address. 
      C:\Users\Trainer>netstat
 
Active Connections
 
Proto  Local Address          Foreign Address        State
TCP    127.0.0.1:39000        PC-114:62701           ESTABLISHED
TCP    127.0.0.1:62394        PC-114:62395           ESTABLISHED
TCP    127.0.0.1:62395        PC-114:62394           ESTABLISHED
TCP    127.0.0.1:62396        PC-114:62397           ESTABLISHED
TCP    127.0.0.1:62397        PC-114:62396           ESTABLISHED
TCP    127.0.0.1:62398        PC-114:62399           ESTABLISHED
TCP    127.0.0.1:62399        PC-114:62398           ESTABLISHED
TCP    127.0.0.1:62402        PC-114:62403           ESTABLISHED
TCP    127.0.0.1:62403        PC-114:62402           ESTABLISHED
TCP    127.0.0.1:62701        PC-114:39000           ESTABLISHED
TCP    192.168.123.50:60906   vs-nwb-cm:http         ESTABLISHED
TCP    192.168.123.50:62717   vs-nwb-file:microsoft-ds  ESTABLISHED
TCP    192.168.123.50:62721   linux800:http          FIN_WAIT_1
TCP    192.168.123.50:62722   text-lb:https          ESTABLISHED
TCP    192.168.123.50:64748   2.18.166.239:https     CLOSE_WAIT
TCP    192.168.123.50:64749   2.18.166.239:https     CLOSE_WAIT

       
      localnet is defined as 127.0.0.0/8
      You can use all the addresses in this range for local purposes…
      In your Linux system, there is a file called /etc/hosts in which some of them are used.”

      127.0.0.1   localhost
127.0.1.1   pc-21 pc21.localhost
 
# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

       

  2. apipa
    If a PC does not receive an address from a DHCP server, even though it is configured to do so, it can generate an address itself (pseudo‑random) in the 169.254.0.0/16 range.
    We call these link‑local addresses or APIPA: Automatic Private IP Addressing.
    These addresses are only usable if they are followed by the network‑interface number, and only if there are other participants in the same network. In practice, you can do very little with them, except quickly diagnosing that there is a DHCP problem. It is generally already difficult to ping other participants…
    ref.: https://en.wikipedia.org/wiki/Link-local_address
     
  3. multicast
     
    All adresses in 224.0.0.0/4 (224.0.0.0 t.e.m. 239.255.255.255) are multicast adresses.
    Multicast can be used by software like dropbox.
     

Common Multicast Examples

Streaming media to multiple receivers
  • IPTV inside a campus or company
  • Live video lectures sent to many classrooms
  • Video conferencing systems that distribute one stream to many participants
    These typically use addresses in the 224.0.0.0/4 range.
Routing protocols

Routers talk to each other using multicast:

  • OSPF → 224.0.0.5 (All OSPF routers) and 224.0.0.6 (Designated routers)
  • RIP v2 → 224.0.0.9
  • EIGRP → 224.0.0.10
Service discovery

Devices discover each other using multicast:

  • mDNS / Bonjour → 224.0.0.251
  • SSDP (UPnP) → 239.255.255.250
    This is how smart TVs, printers, and IoT devices announce themselves.
Multicast DNS (mDNS)

Used heavily on local networks (Apple, Linux, IoT):

  • Resolves names like myprinter.local
  • Uses multicast instead of a central DNS server
    Software distribution / updates

    Some enterprise systems distribute updates via multicast to avoid sending the same file hundreds of times.

Online gaming

Games use multicast for: