A Packet’s First Journey Through a Small Network

This lesson joins the first eight foundations into one complete conversation. You will follow a PC sending an ICMP echo request to a server on another IPv4 network and follow the reply home.

The addresses use documentation ranges, so they explain the decisions without pretending to be your production network. DNS, DHCP and NAT are assumptions or previews here; their full behavior belongs to later lessons.

Before the first frame

The PC already has 192.0.2.10/24, default gateway 192.0.2.1, and destination IP 198.51.100.20. Imagine the address came from configuration or DHCP and the name was already resolved by DNS. Neither service appears in this exchange.

The PC applies its /24 prefix to its own address and the destination. The results differ, so the destination is remote. The PC keeps the server as the IP destination but chooses the default gateway as its first local next hop.

Resolve the next hop

Ethernet cannot send a frame using only the gateway's IP address. When the PC has no cached mapping, it broadcasts an ARP request: “Who has 192.0.2.1?” The destination MAC is FF:FF:FF:FF:FF:FF.

The switch floods that broadcast only inside the local broadcast domain. The router does not forward the ARP request to the server network. Because its LAN interface owns 192.0.2.1, the router replies with that interface's MAC address. The PC records the mapping.

Switch and route the request

The PC builds an Ethernet frame addressed to the router's LAN MAC. Inside it, the IP packet remains addressed from the PC to the remote server, and inside that packet is the ICMP echo request.

The switch reads the destination MAC and forwards the frame toward the router. The router removes the incoming Ethernet frame, reads the destination IP, selects its server-facing interface, decreases TTL, and builds a replacement frame for the next link. The end-to-end IP addresses remain unchanged in this lesson because NAT is not part of the topology.

Return traffic

The server accepts the request and creates an echo reply with the IP endpoints reversed. Its first frame is addressed to its own gateway. At the router, that frame ends; the router selects the PC-side network and creates another frame. The switch forwards it to the PC, completing the round trip.

Complete packet journey

The player starts automatically unless your device requests reduced motion. Pause, move one step at a time, or change speed. Every link names both interfaces. At each step, open the packet and switch between the plain-language and technical inspection views.