Default Gateway

---

• when two end system hosts attempt to communicate via IPv4,
  • protocol compares the src and dst address in each packet against the netmask
  • if the masked portions of the src and dst IP addresses match
    • destination assumed to be reachable via local layer 2 network
      
    • first 28 bits of the src and dst address are the same
    • IP concludes the destination IPv4 address is on the same IP network or subnet
    • tries to deliver packet locally
  • if masked portion does not match
    • IP assumes the packet must be routed to another IP network or subnet:
      
    • when dst IPv4 address is on a different IP network or subnet
      • host forwards packet to its default gateway
        • is a router configured with a path to remote networks
      • router determines what to do with the packet
        • performs same comparison between the src and dst address and netmask
      • router uses its routing table to determine which interface to forward to
        • if not suitable path, router drops the packet
        • informs host that it could not be delivered
      • if packet dst is still another network, process repeats
• paths to other IP networks can be:
  • manually configured in routing table
  • learned by a dynamic routing protocol
    • allow routers to share information about known networks and possible paths to them
    • allows them to choose the best routes to any given destination

Info

Default gateway router’s interface IP can be any usable host ID.

• normally set to first or last usable host address