The internet uses packet switching to transmit data. Instead of sending a message as a single continuous stream (like a phone call on a dedicated circuit), data is broken into small chunks called packets.
Each packet travels independently across the network — possibly via different routes — and is reassembled at the destination.
A packet has two parts:
Some protocols also add a trailer containing error-checking data (e.g. a checksum or CRC — Cyclic Redundancy Check).
| Advantage | Explanation |
|---|---|
| Resilient | If one route fails, packets can take alternative paths — no single point of failure brings the whole network down. |
| Efficient | No dedicated circuit needed — multiple users share the same physical links simultaneously. Bandwidth is used only when data is sent. |
| Scalable | More nodes and links can be added to the network without reconfiguring existing connections. |
| Error handling | Individual packets can be resent if lost or corrupted, without retransmitting the whole message. |
A router is a networking device that forwards packets between networks. It reads the destination IP address in each packet's header and decides the best path to forward it using its routing table.
A routing table is a database stored in a router listing known network destinations and the best path to reach each one. Each entry typically contains:
| Static Routing | Dynamic Routing | |
|---|---|---|
| How configured | Routes manually set by a network admin | Routes learned automatically via routing protocols |
| Adapts to changes? | No — fixed until manually changed | Yes — automatically reroutes around failures |
| Overhead | None at runtime | Routing protocol traffic uses bandwidth |
| Best for | Small, stable networks | Large, complex, changing networks (e.g. the internet) |
Dynamic routing protocols allow routers to share information about network topology and automatically build routing tables:
| Circuit Switching | Packet Switching | |
|---|---|---|
| Connection | Dedicated physical circuit established before communication | No dedicated circuit — packets routed independently |
| Bandwidth | Fixed bandwidth reserved for the duration | Shared — bandwidth used only when data sent |
| Reliability | If circuit fails, communication ends | Packets rerouted if a link fails |
| Latency | Low, consistent (once circuit established) | Variable — packets may queue or take different routes |
| Example use | Traditional telephone network (PSTN) | Internet |
The TTL field in a packet header is a counter that decrements by 1 at each router hop. When TTL reaches 0, the packet is discarded and an ICMP "Time Exceeded" message is sent back to the source. This prevents packets from looping infinitely if there is a routing loop.
The traceroute / tracert command exploits TTL to map the path packets take — it sends packets with TTL=1, 2, 3... and collects the "Time Exceeded" responses from each router along the path.
At the destination, packets are reassembled in the correct order using the sequence numbers in their headers. Packets may arrive out of order (since they can take different routes) or even fail to arrive at all. TCP (at the transport layer) handles acknowledgement and retransmission of missing packets.
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| Term | Definition |
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