Happy Thanksgiving to all you folks out there on a US-based schedule. I’ll keep today’s short and sweet, and blog with you again after the holiday weekend here in the states. As usual, I’ll post the letter answers at the bottom of the post, just so it’s not in the summary, hoping to avoid spoiling the answer if you’ve not seen the question yet.
The question asks about a packet sent by the web server, to the client (PC1), at two different points. For the sake of discussion, let’s say the PC picked dynamic port 1050 as its port number. In this case, the IP addresses and port numbers should be the same at all points in the packet’s path from the server to PC1 – for more background, look at Monday’s post. Specifically, because the question did not state or imply the use of NAT or VPNs, at all points in the route between the server and PC1:
- The source IP address should be the server’s IP address
- The destination IP address should be PC1’s IP address
- The source port number should be 80
- The destination port number should be 1050.
The trickiest part of these four points is the port number part. Because this packet is sent by the server, towards the client, the source port is the well-known port, and the destination port is the port used by the client, in this case 1050. If this doesn’t make much sense still, get a freeware LAN trace tool (Wireshark, formerly Ethereal, is a good one), install it on a PC, turn on a trace, browse a web site, and look at the output. This can be a great training exercise to get your head around what’s flowing in the network.
The MAC addresses do differ based on the location, because the MAC addresses represent address on that local LAN. In this case:
- At point 4, the source MAC is R2’s upper interface MAC, and the destination MAC is R1’s MAC.
- At point 2, the source MAC is R1’s upper interface MAC, and the destination is PC1’s MAC.
Using Monday’s post as the background, when R2 forwards the packet to R1, R2 uses the data link layer to cause the frame to be delivered to R1 – so the frame’s destination is R1’s MAC on that LAN. Similarly, when R1 forwards the frame onto the upper LAN, R1 wants to cause the frame to arrive at PC1, so R1 puts PC1’s MAC address in the frame as the destination MAC address. Finally, note that 802.1Q trunking inserts an additional 4-byte header into the original frame, but it does not change the frame’s MAC addresses, so whether 802.1Q trunking is used or not does not impact which MAC addresses are in use.
Finally, the answers are… B and E.
Happy Turkey day!
