Card 08/ 09
Practice: The Addresses on a Frame at Each Hop
A packet crosses two routers to reach a server; fill in all four addresses on each of three links.
Filling in every address on a frame at every hop is the quickest check that the split between MAC and IP addresses is clear. A laptop sends one packet to a server, through two routers, inside a company network with no NAT. Fill in the source MAC, destination MAC, source IP and destination IP of the frame on each of the three links.
| Device | Card | MAC address | IP address |
|---|---|---|---|
| Laptop | Its only card | 02:00:00:00:00:01 | 10.0.1.20 |
| Router 1 | Laptop side | 02:00:00:00:00:11 | 10.0.1.1 |
| Router 1 | Router 2 side | 02:00:00:00:00:12 | 10.0.2.1 |
| Router 2 | Router 1 side | 02:00:00:00:00:21 | 10.0.2.2 |
| Router 2 | Server side | 02:00:00:00:00:22 | 10.0.3.1 |
| Server | Its only card | 02:00:00:00:00:31 | 10.0.3.30 |
Before you check, say which ARP request each sender had to make on its link.
Show the addresses on each link
| Link | Source MAC | Destination MAC | Source IP | Destination IP |
|---|---|---|---|---|
| Laptop to router 1 | 02:00:00:00:00:01 | 02:00:00:00:00:11 | 10.0.1.20 | 10.0.3.30 |
| Router 1 to router 2 | 02:00:00:00:00:12 | 02:00:00:00:00:21 | 10.0.1.20 | 10.0.3.30 |
| Router 2 to server | 02:00:00:00:00:22 | 02:00:00:00:00:31 | 10.0.1.20 | 10.0.3.30 |
The IP columns never change: they name the two ends. The MAC columns name the two cards on each link, so they change completely at every hop.
The ARP requests: the laptop asked for 10.0.1.1, its gateway, not for the server; router 1 asked for 10.0.2.2, its next hop; router 2 asked for 10.0.3.30, the server, because only router 2 shares a link with it. None of the router IP addresses appears in the packet itself.
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