Follow the Packet, Hop by Hop

About this interactive

An IP packet is a payload with a header in front, and no trailer. The header carries a source IP address and a destination IP address (after 96 bits of other information). The packet is what gets the data to its final destination, and it stays largely the same the whole way. A router routes packets. It reads the destination IP address, decides the next best step toward it, and sends the packet on. The next router does the same, and so on: each step is a hop. tracert, on Windows, lists them: one line for each router that answers along the way, and a last line for the destination itself. To cross each link, the packet is put inside a frame, which carries the local addresses: on Ethernet, a destination and source MAC address. A computer sending to a machine on another network addresses the frame to its router, the default gateway, not to the far machine. That router strips the frame off, reads the packet, and builds a new frame addressed to the next router. Every router does the same. So one packet is carried by a new frame on every link: a path with three routers has four links, and four frames. So: Ethernet (Layer 2) uses MAC addresses, calls its unit a frame, and is forwarded by switches (a hub only repeats signals, at Layer 1). IP (Layer 3) uses IP addresses, calls its unit a packet, and is routed by routers. LANs sit at each end, and the links between routers are WAN links. They may not even be Ethernet, which is why every item here says when they are. Two things in a packet do change on the way, as the lesson admits: each router lowers its time-to-live count by one, and some routers rewrite the source address (NAT), which the packet journey animator after this lesson shows. The destination IP address does not change.

What you'll learn

Follow a packet hop by hop and name the destination MAC and destination IP on any link: the destination IP stays the final destination's, the destination MAC is the next device's
Count what a path involves: the frames that carry the packet, the routers that read it, the LANs and WAN links it crosses, and the lines tracert prints for it

Aligned to

ISC2 CC

4.1 Understand network security

CompTIA Network+

1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.

1.6 Compare and contrast network topologies, architectures, and types.

Cisco CCNA

3.2 Determine how a router makes a forwarding decision by default

Cisco CCST Networking

1.3 Differentiate between LAN, WAN, MAN, CAN, PAN, and WLAN

3.4 Demonstrate how to set up and check network connectivity

Key terms

Packet: A unit of data formatted for transmission over a network, containing a header, payload, and sometimes a trailer.
Frame: The unit of data used at Layer 2 (Data Link layer) of the OSI model, containing source and destination MAC addresses, a payload (the encapsulated Layer 3 packet), and a frame check sequence for error detection. Frames are how switches and other Layer 2 devices think about and forward data.
Router: A network device that forwards data packets between networks based on IP addresses.
Hop: A single step in the path a packet takes through the network, specifically the transit from one router to the next closest router toward the destination. The number of hops a packet must travel is used as a routing metric by distance-vector protocols like RIP.
Layer 2: The Data Link layer of the OSI model, responsible for MAC addressing and frame delivery on a local network.
Layer 3: The Network layer of the OSI model, responsible for logical addressing and routing packets between networks.
IP Packet Header: The portion of an IP packet that contains control information, including the source and destination IP addresses, prepended to the payload.
Wide Area Network: A network that spans a large geographic area, connecting multiple local area networks.
tracert: A Windows command-line tool that traces the path packets take to a destination, displaying each hop and its response time to help identify routing delays or failures.
Default Gateway: The router that a device uses to send traffic to destinations outside its local network.
Local Area Network: A network that connects devices within a limited geographic area such as a home, office floor, or building, where all devices share the same broadcast domain and communicate at high speeds with low latency. A router defines the boundary of a LAN by separating it from other networks.
Encapsulation: The process of wrapping data with protocol headers as it passes down the layers of the OSI model.

Topics

Packet Frame Router Hop Layer 2 Layer 3 Interactive Fill In

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