IPv4 Forwarding

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• When a host attempts to send a packet via IPv4
  • the protocol compares the source and destination IP address in the packet against the sending host’s subnet mask
    • If the masked portions of the source and destination IP addresses match, then the destination interface is assumed to be on the same IP network or subnet
    • E.g.,:

Example

• host will determine that the destination IPv4 address is on the same IP network (192.168.0.0/24) and try to deliver the packet locally
• On Ethernet, the host would use the address resolution protocol (ARP) to identify the MAC address associated with the destination IP address

• If the masked portion does not match, the host assumes that the packet must be routed to another IP network
  • E.g.,

Example

• source host 192.168.0.100 identifies that the destination IPv4 address is on a different IP network (192.168.1.0/24)
• it forwards the packet to a router rather than trying to deliver it locally
  • Most hosts are configured with a default gateway parameter
    • is the IP address of a router interface that the host can use to forward packets to other networks
  • default gateway must be in the same IP network as the host