Path of a Packet

A packet path is a sequence of local decisions. The source host, each router, and the destination apply their own routing, neighbor, filtering, and protocol state; no endpoint normally knows every internal decision in advance.

Sending to an On-Link Destination

For a destination covered by a connected route, the source selects an interface and source IP. It then resolves the destination's link address—ARP for IPv4 over Ethernet or Neighbor Discovery for IPv6—and sends a frame carrying the IP packet. A switch can forward the frame without becoming an IP hop.

Sending Through a Gateway

For an off-link destination, the selected route identifies a next-hop router. The IP destination remains the remote endpoint, while the local frame destination is the gateway's link address. The host resolves the gateway, not the remote server, on its local link.

Processing at Each Router

A router removes incoming link framing, validates and processes the IP header, decrements TTL or Hop Limit, looks up the destination, applies policy, and creates new framing for the outgoing link. For IPv4, header checksum processing reflects the changed TTL. If the hop field reaches zero, the router drops the packet and can return an ICMP time-exceeded message.

Accounting for Middleboxes and MTU

Ordinary routing preserves source and destination IP addresses, but NAT can rewrite them and tunnels can wrap the original packet. Firewalls can drop traffic silently or reject it. Link MTUs also differ; IPv4 routers can sometimes fragment packets, while IPv6 routers do not fragment forwarded packets and rely on Path MTU Discovery.

Following the Return Path

The destination performs its own route lookup for the response. The return path can use different routers due to routing policy, load balancing, or failures. Stateful firewalls and NAT must account for the observed flow, so asymmetry can matter operationally even when IP permits it.

Summary

You can now trace the changing link state around a routed IP packet.

1. Resolve the final host only when it is on-link.
2. Frame off-link traffic to the selected local gateway.
3. Follow route lookup and hop-limit processing at each router.
4. Account for NAT, filtering, tunnels, and MTU constraints.
5. Treat the return direction as an independent route.