The Application Layer

Lesson 22 of 32 in Coddy's The OSI Model course.

Layer 7 is where the protocols you have heard of live. Not the applications themselves, but the languages they speak:

HTTP: fetching and submitting web content
DNS: turning names like coddy.tech into IP addresses
SMTP: moving email between servers
SSH: secure remote terminals

Each rides on the lower layers: HTTP over TCP over IP over Ethernet. When someone says "an L7 problem", they mean the machines can talk fine, but the conversation is wrong: a 404, a bad query, a misconfigured server.

Challenge

Beginner

Print the layer 7 protocol for each job, one per line, uppercase:

Loading a web page
Sending email between servers
Resolving a hostname to an IP

Expected output:

HTTP
SMTP
DNS

Try it yourself

Terminal

All lessons in The OSI Model

1Why Layers

The Layering Idea Seven Layers Numbers and Names Recap - The Stack

4The Network Layer

IP and TTL Ping and ICMP Routing Tables Recap - Layer Three

7Encapsulation

Wrapping Data Headers Everywhere Reading a Capture Recap - Follow the Wrap

2The Physical Layer

Bits on a Wire How Long Will It Take Duplex and Collisions

5Transport: TCP and UDP

Ports The TCP Handshake TCP vs UDP Sockets in Action Recap - Layer Four

8The Journey of a Packet

A Click Happens Crossing the Networks Arriving and Climbing Recap - End to End

3Data Link and MACs

Frames and MACs Reading a MAC ARP Recap - Layer Two

6The Upper Layers

Session and PresentationThe Application LayerWatching the Layers Recap - Top of the Stack

Practice on your own: Terminal playground