Port numbers

Unanswered Question
Mar 9th, 2007

Hi All,

While reviewing the IANA port number assignment:


I've noticed a weird phenomena, FTP for example as we all know uses TCP ports 20 and 21, in the document it is stated that it uses both TCP and UDP, most of the protocols have this symptom, is there any logical explanation for this ?

Thanks in advance,

Mohammed Mahmoud.

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mohammedmahmoud Fri, 03/09/2007 - 09:34

Hi Medan,

Thanks a lot, the aim of my question was when permitting or denying a certain protocol by denying its port number, will i have to deny both TCP and UDP.

Thanks in advance,

Mohammed Mahmoud.

Danilo Dy Fri, 03/09/2007 - 10:05

Well, Firewalls/ACL's have a default deny any any at the bottom of the rules. Even you don't put a rule to block them, it well be block. But some ports have to block on top or in the middle of the Firewall rules or ACL, you should deny their well known port and protocol unless otherwise advise.

The TCP/UDP is the programmers choice of protocol to be use by its program depends on its architecture and/or operations. Some put a configurable feature for the user to choose between TCP and UDP

mohammedmahmoud Fri, 03/09/2007 - 10:21

Thanks Medan,

But my question is simply for example, for me to assure denying FTP do i have to deny both its TCP and UDP ports, and the same for every protocol that supports both TCP and UDP as stated by IANA ?

Thanks in advance,

Mohammed Mahmoud.

Danilo Dy Fri, 03/09/2007 - 10:55

Nope, no need to block both. Just block TCP will do. Same for other ports, just block their protocol - TCP or UDP, not TCP and UDP

jhimye Fri, 03/09/2007 - 13:27

Just want to add....

I don't know how many services are like DNS, but it uses both TCP and UDP on port 53, but for different functions. UDP for lookups, and TCP for zone transfers. FYI...


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