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.
Routing · Lesson 3
Path of a Packet
Learn how routes, neighbor discovery, frames, and routers carry an IP packet across a path.
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.
Does an ordinary Ethernet switch count as an IP routing 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.
Whose MAC address is used in the first Ethernet frame to an off-link server?
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.
Which IP field is changed by every normal routed hop?
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.
When might end-to-end IP addresses change along a path?
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.
Must a reply traverse the same routers in reverse order?
Lesson complete
You finished Path of a Packet
You can now trace the changing link state around a routed IP packet.
Resolve the final host only when it is on-link.
Frame off-link traffic to the selected local gateway.
Follow route lookup and hop-limit processing at each router.
Account for NAT, filtering, tunnels, and MTU constraints.
Treat the return direction as an independent route.
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