Understand switch frame forwarding and router packet forwarding.
Protocol layers and data encapsulation
An application creates data. Transport protocols help applications exchange it. IP provides addressing between networks, and Ethernet or Wi-Fi carries it across a local link. Each layer adds information the next device needs. Adding protocol headers, and trailers where applicable, is called encapsulation.
The OSI model names seven layers: physical, data link, network, transport, session, presentation and application. Use it as a troubleshooting aid: check the cable, local connection, IP path and application. Protocol implementations do not always map directly to individual OSI layers.
TCP and UDP
TCP provides an ordered byte stream and mechanisms for retransmission and flow control. Establishing a TCP connection uses a handshake. UDP sends separate datagrams without TCP’s delivery and ordering mechanisms; an application may add its own reliability. “UDP is always faster” is too simple: the application and conditions matter.
Transport port numbers identify application endpoints on a host. DNS commonly uses port 53 over UDP or TCP. HTTPS commonly uses TCP 443; HTTP/3 uses QUIC over UDP. An IP address identifies a network destination, while a port helps identify the service there. A successful ping tests ICMP reachability, not whether a web service is working.
Practical exercise
A server answers ping but its web page will not open. Name two checks beyond repeating the ping.
Show the answer and explanation
Check that the application is running and listening on the intended port, and that a firewall allows that port. Also check DNS, the requested URL and TLS errors when applicable.
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Technical references checked on 25 October 2026. These are original teaching explanations, not official Cisco course material. See how content and lab validation are described.
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