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Protocol Explorer · FTP

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

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PROTOCOL EXPLORERLearn how the internet works
Now learningFTP · File transferOverview · 5 steps
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Web Server203.0.113.10InternetRouter192.168.1.1Client192.168.1.10

Protocol overview

FTP

Active

File Transfer Protocol

Separate a persistent command connection from a data connection. Follow a passive-mode file download and interpret FTP reply classes.

Layer
Application (Layer 7)
Default port
21 control; separate data connection
Transport
TCP
Data format
Text control + separate data channel
Defined in
RFC 959, RFC 2428
Read the complete guide ↗

Packet flow

DNSTCPRequestResponse
No.TimeSourceDestinationProtocolInfoSize
10.000s203.0.113.10192.168.1.10FTP220 FTP ready69 B
20.024s192.168.1.10203.0.113.10FTPUSER learner68 B
30.048s203.0.113.10192.168.1.10FTP331 Password required77 B
40.072s192.168.1.10203.0.113.10FTPPASS teaching-only74 B
50.096s203.0.113.10192.168.1.10FTP230 Logged in69 B
60.120s192.168.1.10203.0.113.10FTPTYPE I62 B
70.144s192.168.1.10203.0.113.10FTPEPSV60 B
80.168s203.0.113.10192.168.1.10FTP229 Entering Extended Passive Mode (|||50000|)102 B
90.192s192.168.1.10203.0.113.10TCP52344 → 50000 [SYN] Seq=066 B
100.216s203.0.113.10192.168.1.10TCP50000 → 52344 [SYN, ACK] Seq=0 Ack=166 B
110.240s192.168.1.10203.0.113.10TCP52344 → 50000 [ACK] Seq=1 Ack=160 B
120.264s192.168.1.10203.0.113.10FTPRETR demo.txt69 B
130.288s203.0.113.10192.168.1.10FTP150 Opening data connection83 B
140.312s203.0.113.10192.168.1.10FTP-DATAFile bytes on passive data connection76 B
150.336s203.0.113.10192.168.1.10FTP226 Transfer complete77 B
160.360s192.168.1.10203.0.113.10FTPQUIT60 B

NAVSPROUT · PROTOCOL FIELD GUIDES

FTP, packet by packet.

Separate a persistent command connection from a data connection. Follow a passive-mode file download and interpret FTP reply classes.

PROTOCOL FIELD GUIDE

How FTP works

Separate a persistent command connection from a data connection. Follow a passive-mode file download and interpret FTP reply classes.

Default port
21 control; separate data connection
Transport / carrier
TCP
Category
application

1. Open the control channel

The client connects to port 21 and receives 220. USER and PASS authorize the session; classic FTP exposes these commands in plaintext.

2. Choose a representation

TYPE I selects binary transfer. Text mode can transform line endings, so binary mode matters for arbitrary files.

3. Negotiate passive data

EPSV tells the client which server port to connect to for data. The control connection stays open while a second TCP connection carries file bytes.

4. Retrieve a file

RETR starts the transfer. A 150 reply precedes the data; 226 confirms transfer completion on the control channel.

5. Close the session

QUIT ends control communication. Passive FTP differs from active mode, in which the server opens the data connection toward the client.

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 959 ↗ RFC 2428 ↗

Check your understanding

What carries file bytes in passive FTP?

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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