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NAT

  • network address translation - meant to address problems of running out of IPv4 address space

  • inside/outside - (literally) which device is being translated by NAT or is the destination

  • local/global - (perspective) - inside/outside network - is it local to you?

  • inside local - source address seen from inside network (gen. private IPv4)

  • inside global - source address seen from outside network (gen. globally routable IPv4)
  • outside local - dest address seen from inside network (gen. globally routable IPv4)

  • outside global - dest address seen from outside network (gen. globally routable IPv4)

    • if it ever asks about an outside host ip, assume its this one

  • NAT64 translates IPv6 to IPv4, no private/public address translation

types of NAT

  • static NAT - one-to-one mapping of local and global addresses

  • dynamic NAT - pool of public adds -> assign on queue basis (if run out of addresses -> next request fails)

  • PAT (Port Address Translation/NAT overload) - mult. private IPv4 to few/single public IPv4

    • done w src port numbers from TCP/IP session (port nums added to inside global adds) or smth else if packet doesn't involve TCP/UDP (e.g. ICMP uses Query ID)
    • may not translate to original port at the router bc it may already have been used -> uses next available port
    • most home routers, small businesses
      • doesn't need enough public adds for total user sessions

  • [[port forwarding]] - one public IP w port numbers representing different devices in LAN

Behavior

  • dynamic
    • inside to outside - router receives packet (ip nat inside) from inside network
      1. check the associated ACL, is it permitted? -> yes, need to translate
      2. check NAT entries -> no entry so needs to be translated
      3. uses next available add in pool and creates translation entry
      4. replaces inside local address w new inside global address and forwards packet to outside network
    • outside to inside - router receives packet from outside network (ip nat outside)
      1. when receive packet, does NAT table lookup
      2. forwards it to inside PC

problems

  • still ran out of addresses anyways
  • geographical distribution (50% of used IP's in US)
  • use of NAT breaks end-to-end onnectivity