To start, let's get one thing clear. With TCP traffic there are always TWO ports involved, yours and mine; the client's and the server's.
Imagine I'm the client. And I'm calling a website (invoking a web server). I'll send in an HTTP request. With nothing 'special' on the url, I'll be calling the server's port 80. From one of 'my' ports. My port will be an Ephemeral one (literally).
So:
Me You
'50001' ==> 80
I initiate the request on an ephemeral port, targeting a specific server port. You respond to me on that open (ephemeral) port.
Active FTP
Now, let's consider FTP Active mode.
This works in the same sort of way. But FTP uses two ports. One for the Commands (logging on, request a file, and so on) and another to ship data on. However, first the Command line is established.
Like HTTP, FTP will generally use a fixed port, 21, for the initial Command channel.
So:
Me You
'50001' ==> 21
I initiate the request on an ephemeral port, targeting a specific server port. You respond to me on that open (ephemeral) port.
So far, so like HTTP.
Now, I ask for a file from You - a 'get' request. In Active mode I start this off by sending you a PORT command. This tells you which port to use my side for the data transfer. I'm being 'Active' and telling you which port to open my side.
Me You Content
'50001' ==> 21 PORT a,b,c,d,p1,p2
(the port command is my IP address a.b.c.d and the port to use which is found as (256 x p1) + p2)
At this point, I've told You (the client has told the server) to open a port on the client, to use as the Data channel. So the Server then opens that port on the Client, and sends the data down.
Me You
56001 <== '32001'
The difference here is, although I've told you which port my side to use, it's You who initiates the transfer; you open the port on my side. Of course, like any 'client' (You are acting like a client now), You use an ephemeral port your side, to open that specific (although it'll be one of my ephemeral ones) port my side.
It's 'Active' as I'm the one controlling Port usage. The ephemeral bit is always the same - the client picks an outgoing port from the ephemeral pool, but always target a specific port on the server you call. Initially, and always, for the Command channel it's the standard you've agreed (typically, 21), but the Data channel will be an ephemeral one on the Client side, even though it's being specifically requested by the FTP server (on the advice of the active FTP client).
And this is the "issue" with Active FTP...
Consider HTTP traffic ... HTTP client (web browser) targets server port 80 of web server, using an ephemeral client port; server responds to request;
Consider SMTP traffic ... SMTP client (email agent) targets server port 587 of mail server, using an ephemeral client port; server responds to request;
If you're a firewall watching this network traffic, the client is always targeting known, specific remote targets; it's always initiating the request; it's always getting the response back on the open channel that it started.
But for FTP, although the Command channel follows that model, the Data channel doesn't. The firewall sees all these (seemingly) unsolicited requests coming in on non-standard ports, to non-server computers. It's the opposite direction to 'normal' traffic; and firewalls don't like that. This is why Active FTP can seem to work for a while, then stop when your firewall gets interesting.
Passive FTP
This options attempts to make FTP data transfers work in the same way as normal TCP traffic. The client always the client; the server always the server.
The Command channel works in the same way, but when a file is requested by the client, the response from the Server this time is You telling me which port to use on the Server.
So:
Me You Content
'50001' ==> 21 get file
The response to this request is You saying which port to use to fetch the file on. The port on the Server.
Thus:
Me You
'50002' ==> 36001
i.e. the response gives Me the number of one of your ephemeral ports, which I then target specifically (with one of my ephemeral ports of course). In the response to this command, I get the file I want. I'm being 'passive'; You are telling Me which port on Your side to use. The direction of data flowing now matches the same profile as all other TCP standards, and no firewalls get anxious about things (well, the server needs to set aside a range of its ephemeral ports to pick from, and these are usually quite high, well away from any 'interesting' ones).
And there you have it :) Use Passive FTP. You are being passive, ports are selected by the server, but it means that data flows in a standard manner.
ADDENDUM
With a Linux machine you can use the netstat command to see the current state of your TCP connections. This makes clear both the difference between My and Your port numbers, and shows which connections are currently in use:
netstat -an
tcp 0 0 192.168.0.7:45619 176.255.246.57:80 TIME_WAIT
tcp 0 0 192.168.0.7:33474 212.73.212.117:80 ESTABLISHED
tcp 0 0 192.168.0.7:36383 176.255.246.56:80 TIME_WAIT
tcp 1 0 192.168.0.7:50603 77.67.21.25:80 CLOSE_WAIT
tcp 0 0 192.168.0.7:53067 176.255.246.72:80 TIME_WAIT
tcp 0 0 192.168.0.7:58593 74.125.230.207:443 ESTABLISHED
tcp 0 0 192.168.0.7:59389 176.255.246.40:80 TIME_WAIT
tcp 0 0 192.168.0.7:47264 199.59.150.41:443 ESTABLISHED
tcp 0 0 192.168.0.7:57111 74.125.132.125:5222 ESTABLISHED
tcp 0 0 192.168.0.7:47645 63.135.90.55:80 ESTABLISHED
tcp 0 0 192.168.0.7:59765 66.196.66.212:80 ESTABLISHED
tcp 1 0 192.168.0.7:54573 90.223.233.145:80 CLOSE_WAIT
tcp 0 0 192.168.0.7:38578 64.13.137.78:80 TIME_WAIT
tcp 0 0 192.168.0.7:59410 173.194.41.135:443 ESTABLISHED
The left-hand figures are My ports ... all in the ephemeral range as I've initiated the connection; Your ports are all standard numbers for what they are (80 being HTTP and 443 for HTTPS).
