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Protocol Explorer · POP3

Explore internet protocols with animated messages, real arrival order and packet inspection.

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PROTOCOL EXPLORERLearn how the internet works
Now learningPOP3 · Retrieving emailOverview · 4 steps
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0 / 14 complete · 0 in progress
Mail Server198.51.100.25InternetRouter192.168.1.1Client192.168.1.10

Protocol overview

POP3

Active

Post Office Protocol version 3

Retrieve messages from a mailbox through authorization, transaction and update states. POP3 has a simpler mailbox model than IMAP.

Layer
Application (Layer 7)
Default port
110 / 995 implicit TLS
Transport
TCP
Data format
Text commands + multiline replies
Defined in
RFC 1939
Read the complete guide ↗

Packet flow

DNSTCPRequestResponse
No.TimeSourceDestinationProtocolInfoSize
10.000s198.51.100.25192.168.1.10POP3+OK mailbox ready73 B
20.024s192.168.1.10198.51.100.25POP3USER learner68 B
30.048s198.51.100.25192.168.1.10POP3+OK60 B
40.072s192.168.1.10198.51.100.25POP3PASS teaching-only74 B
50.096s198.51.100.25192.168.1.10POP3+OK authorized70 B
60.120s192.168.1.10198.51.100.25POP3STAT60 B
70.144s198.51.100.25192.168.1.10POP3+OK 1 5864 B
80.168s192.168.1.10198.51.100.25POP3RETR 162 B
90.192s198.51.100.25192.168.1.10POP3+OK message follows118 B
100.216s192.168.1.10198.51.100.25POP3DELE 162 B
110.240s198.51.100.25192.168.1.10POP3+OK marked66 B
120.264s192.168.1.10198.51.100.25POP3QUIT60 B
130.288s198.51.100.25192.168.1.10POP3+OK updated67 B

NAVSPROUT · PROTOCOL FIELD GUIDES

POP3, packet by packet.

Retrieve messages from a mailbox through authorization, transaction and update states. POP3 has a simpler mailbox model than IMAP.

PROTOCOL FIELD GUIDE

How POP3 works

Retrieve messages from a mailbox through authorization, transaction and update states. POP3 has a simpler mailbox model than IMAP.

Default port
110 / 995 implicit TLS
Transport / carrier
TCP
Category
application

1. Authorize mailbox access

The server sends +OK. The classic USER/PASS exchange shown here is readable teaching data; protected connections are needed for real passwords.

2. List available messages

STAT reports message count and total size. LIST reports individual message numbers and octet counts in the current maildrop.

3. Retrieve a message

RETR transfers a message including headers and body. Multiline replies end with a single-dot line and use dot-stuffing.

4. Commit pending deletions

DELE marks a message for deletion. Successful QUIT enters the update state; retrieving a message alone does not automatically delete it.

Protocol relationships

Protocols cooperate across layers. Compare the neighboring lessons to distinguish addressing, delivery, application semantics and encryption.

Standards and editorial method

Written and implemented independently by NavSprout. Lessons summarize public protocol standards using synthetic documentation addresses and fictional credentials. Consult the standards for complete wire formats and normative behavior.

RFC 1939 ↗

Check your understanding

What does DELE do before QUIT?

Why it’s hereNAVSPROUT ORIGINAL

Reading definitions is easier when you can connect them to a packet. This first-party learning tool pairs static explanations with a visual lab, byte inspection and a short knowledge check. All course data and application resources ship with NavSprout.

  1. Choose DNS and follow the lookup from client to authoritative server.
  2. Switch to the sequence view, then inspect a packet and select a field to highlight its bytes.
  3. Compare TCP with UDP, answer the quiz and export your progress.

Frequently asked questions

What happens when I open a website?

A typical first HTTPS visit can involve DNS resolution, a transport connection, a TLS handshake and an HTTP exchange. Caches, connection reuse and HTTP/3 can change this sequence.

How are TCP and UDP different?

TCP presents a reliable ordered byte stream. UDP preserves individual datagram boundaries without built-in delivery guarantees. Applications choose the behavior they need.

Are these real captured packets?

No. Frames are generated locally for teaching. Header lengths and checksums are calculated, while TLS ciphertext and some negotiation payloads are illustrative. Downloaded PCAP files contain synthetic examples, not live traffic.

Can I learn without JavaScript or an account?

All 14 protocol guides and their lesson explanations are available as ordinary HTML without signing in. JavaScript powers animations, byte inspection, local progress and quizzes.

Where is my progress saved?

Only in this browser’s local storage. Export a progress JSON file to move it to another device. Clearing browser data can remove progress.

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