<OT> what is a port?

[vu pham]

vu pham wrote:
> Tony Alfrey wrote:
>> vu pham wrote:
>> <snip>
>>>
>>> Personally I think it is the mail server owners are the people who 
>>> are responsible for blocking spam mails, not the ISP. The ISP may own 
>>> mail servers, but they should just limit the spam mails by setting up 
>>> filters, rules on their mail servers, not on the network.
>>
>> Ah, yes, that would be convenient but I think my chances of getting 
>> Comcast to do that are zero.  So it sounds like you are saying that 
>> Comcast can limit traffic that includes access to port 25 
>> *independent* of actually accessing the Comcast smtp server?
>>
> 
> I believe the ISPs, if they want, can block any packets no matter where 
> those packets go to, as long as those packets pass thru their routers.
> 
> Hey, you go thru *my* house, you have to take my rules :)

I would like to add a little bit here : destination
can be defined by combination of ip address ( which host), port( which 
service), and status ( ack packet,syn/ack packet ... in tcp, or service 
type in icmp ... ).

So, they can do things like : Hmm, Mr Tony complains our ISP :) we know 
his modem has the ip 1.2.3.4. Now, just put the filter on our router 
that any packet that has  1.2.3.4 in the ip source field, and has 25 in 
the destination port field will be rejected. Then you cannot send emails 
to anybody from your home using smtp :)))

Vu