CCNA Networking · Quick Reference

Module 15 — Application Layer Cheat Sheet

CCNA Networking

In one line: The top three layers of the OSI model work together to let software talk to the network in a way that's meaningful to humans and applications, not just bits and bytes.

Key Ideas

1Application, Presentation, and Session. The top three layers of the OSI model work together to let software talk to the network in a way that's meaningful to humans and applications, not just bits and bytes.

2Peer-to-Peer Networks and Applications. Most networks use a client-server model — a server provides centralized resources, and clients request them. Peer-to-peer (P2P) flips this: every device can act as bot...

3Web and Email Protocols. A handful of Application Layer protocols power most of what people think of as "the internet" day-to-day.

4IP Addressing Services. Two Application Layer protocols quietly handle the addressing work that makes the rest of networking usable for humans: DNS translates names to addresses, and DHCP ass...

5File Sharing Services. FTP (File Transfer Protocol) is the classic protocol for transferring files between a client and server. Unlike most Application Layer protocols, FTP uses two separate...

Code Examples

! User types a domain name in the browser
www.example.com

! Client queries its configured DNS server (often via UDP port 53)
Client → "What is the IP for www.example.com?" → DNS Server

! DNS server replies with the resolved IP address
DNS Server → ...

1. DISCOVER — Client broadcasts: "Is there a DHCP server out there?"
Client → 255.255.255.255 (broadcast)

2. OFFER — Server replies with an available IP address offer
Server → Client: "I can offer you 192.168.1.50"

3. REQUEST — Client formally reque...