Trace a packet across a router

Trace a packet across a router — guided practice

Follow source/destination IPs and next-hop forwarding without changing the intended destination.

Use network-notes.md or paper. All addresses and outputs are fictional. Room A has laptop A at 192.0.2.10/24, printer P at 192.0.2.20/24 and a switch. Router R has A-side 192.0.2.1/24 and B-side 198.51.100.1/24. Room B has server S at 198.51.100.20/24 and its switch. A uses default gateway 192.0.2.1. A supplied DNS record maps catalog.example to 198.51.100.20. For this /24 exercise, matching first three octets means same subnet. No live network access, capture, scanning or device configuration is needed.

1. 1. Write the packet card

   Source IP=192.0.2.10; destination IP=198.51.100.20; payload=request for catalog.

   You should see: A and S are the end hosts.

   The destination is the server, not the gateway.

2. 2. Choose A’s next hop

   Use the /24 rule and A’s gateway.

   You should see: A sends the packet toward 192.0.2.1.

   A knows the router can reach beyond the local subnet.

3. 3. Use R’s route card

   R has direct route 198.51.100.0/24 through its B-side.

   You should see: R forwards toward S on Room B’s local network.

   Each router uses its own routing information.

4. 4. Track a hop limit

   For a simplified IP card with TTL 5 at A, subtract one at R.

   You should see: TTL leaving R is 4.

   A hop limit prevents endless circulation; no NAT is modeled here.