How a packet travels from one device to another

Every message you send — an email, a web page, a ping — is chopped up, wrapped in layers, and carried across cables and switches and routers to reach its destination. This site shows you the whole journey, step by step, using the OSI model and real Cisco terminology.

L1–L7 The OSI Model

The 7-layer model that every network engineer uses to describe how data is prepared, carried, and delivered.

Explore the 7 layers

Same network Host to Host (LAN)

Two PCs on the same subnet. Follow the data down one host's stack, through a switch, and up the other's — no router needed.

Watch the journey

Different networks Across the internet

Two PCs in different subnets. Now the data needs a router (default gateway), a routing table, and lots of de-encapsulation.

Watch the journey

The big idea

When you send data, each layer of the OSI model adds a header (and sometimes a trailer) to the data from the layer above. That is called encapsulation. The receiving device then strips those headers off one by one — de-encapsulation — until the original data is handed to the application.

Data → Segment → Packet → Frame → Bits

Each step down adds one more wrapper. The official name for "data at a given layer" is the Protocol Data Unit (PDU): data (L7–L5), segment (L4), packet (L3), frame (L2), and bits (L1).

Cisco terms you will meet

Cisco spin

Cisco devices run an operating system called IOS. Switches are Layer 2 devices that forward frames using MAC addresses. Routers are Layer 3 devices that forward packets using IP addresses and a routing table. The address of the router your device sends to when the destination isn't local is your default gateway.

Encapsulation

Adding a header at each layer as data travels down the stack. Like putting a letter inside envelopes, each labeled differently.

De-encapsulation

Removing headers as data travels up the destination's stack, layer by layer.

PDU

Protocol Data Unit — the generic name for data at any layer (data, segment, packet, frame, bits).

MAC address

Hardware address burned into a NIC. Layer 2 addressing, used to deliver a frame across one link. Written like 00:1B:44:11:3A:B7.

IP address

Logical address that identifies a device on a network, e.g. 192.168.1.10. Layer 3 addressing, used to route across networks.

Subnet mask

Tells a host which part of an IP is the "network" part. 255.255.255.0 means the first three octets identify the network.

Default gateway

The router's IP address on your subnet. Traffic to other networks is sent here first.

ARP

Address Resolution Protocol. Asks "who has this IP?" so the sender can learn the MAC address to put in the frame. IPv6 uses NDP instead.

ARP table

A cache on every host and router mapping IP addresses to MAC addresses (Cisco: show arp).

MAC address table

The switch's map of which MAC address is reachable out of which port (Cisco: show mac address-table).

Routing table

The router's map of which networks are reachable via which interface/next hop (Cisco: show ip route).

Hop

Each router a packet passes through is one hop. TTL (Time To Live) counts hops down from 64 or 255 to stop packets looping forever.

Unicast / broadcast

Unicast = frame to one specific MAC. Broadcast = frame to everyone (MAC FF:FF:FF:FF:FF:FF), used by ARP.

VLAN

Virtual LAN — a switch can split one physical network into several separate Layer 2 networks. Broadcasts stay inside the VLAN.

Where to start

New to networking? Read The OSI Model first — it gives you the vocabulary. Then watch the same-network journey, and finally the cross-network journey to see how routers stitch networks together.