Packets and Frames
Asha sends a 3 MB mock-up to a client overseas. It does not travel as one 3 MB blob. It is sliced into a few thousand small pieces, and each piece is wrapped twice: once for the whole journey, and once for the single link it is crossing right now. Knowing which wrapper is which is the key to reading any network trace.
The rule
The packet carries the journey; the frame carries one link.
Packet. A packet is a chunk of data plus an IP header. That header holds the source and destination IP addresses, the addresses of the two ends of the whole journey. Asha's laptop and the client's server are named here, and they stay the same all the way across the internet. (You will sometimes see these called "packages" in casual writing; the engineering word is packet.)
Frame. To cross one physical link — a cable to the switch, the Wi-Fi hop to the access point — the packet is placed inside a frame. The frame header holds MAC addresses, the hardware addresses of the two devices on either end of that link only. A frame also ends with a checksum so the receiver can spot a corrupted frame and drop it.
Size limit. A link can carry frames only up to a certain size. On ordinary Ethernet the packet inside a frame may be at most 1500 bytes; this limit is the MTU (maximum transmission unit). That is why big files are split into many packets.
The shortcut
Every time a packet reaches a router, picture it changing envelopes: the router tears off the old frame, reads the packet's destination IP, and puts the packet into a fresh frame for the next link. So, for any hop, ask two questions: "Where is this ultimately going?" → read the packet. "Who is the next device on this wire?" → read the frame.
Build the packet
Source IP: Asha's laptop. Destination IP: the client's server. These two will not change on the way.
The classic trap: thinking the destination MAC on Asha's first frame is the overseas server's MAC. Her laptop never learns that address; it only ever needs the MAC of the next device on its own link.
Drills
Answer fast — trust the rule.
Which addresses are in the IP packet header?
A frame's MAC addresses describe…
Asha's first frame leaves her laptop for a server overseas. Its destination MAC belongs to…
On standard Ethernet, what is the largest packet a frame can carry?
As a packet crosses five routers, what normally stays the same?
Remember
- The packet carries the journey (IP addresses); the frame carries one link (MAC addresses).
- At every router, picture an envelope change: same packet, new frame.
- The first frame goes to the router's MAC, never to the remote server's.