Re: TCPIP Port Communication

From: Steven L Umbach (n9rou_at_nospam-comcast.net)
Date: 01/12/04


Date: Mon, 12 Jan 2004 01:13:04 GMT

The two computers engaging in a tcp/ip session use a source and destination port. The
services you mention use "well known ports" while the client computer usually uses a
random unprivileged port in the above 1023 range. The server computer can have many
connections on the same port, but from different source ip addresses/client port
combinations. If you look at netstat -an on your computer while connected to the
internet, you will see this connection between your computer and the web server. The
session will show a source and destination address such as your computer
xxx.xxx.xxx.xxx:1138 and destination as xxx.xxx.xxx.xxx:80. The numbers after the
colon are the port numbers involved with the connection. I am not an expert on this,
but I know at least TCP uses sequence numbers in the packets exchanged so that
packets do not have to arrive in any particular order and to give some degree of
security to the connection. A stateful inspection firewall will also keep track of
the "state" of a connection with parameters that include sequence numbers and ports
used in the nat/pat process to make it much more difficult for a hacker to try to
hijack a tcp/ip session. --- Steve

http://support.microsoft.com/default.aspx?kbid=172983
http://www.robertgraham.com/pubs/firewall-seen.html#1
http://www.ssimail.com/Stateful.htm
http://www.netscreen.com/products/firewall/security/stateful_inspection.jsp

"Todd" <anonymous@discussions.microsoft.com> wrote in message
news:5864136A-98E0-4CC6-8430-3FDA05750728@microsoft.com...
> When using IP does all communication go over the same ports......??
> eg When a SMTP connection is established on port 25 , does all communication happen
on this one port ? What happens when another connection comes in doesn this also
communicated over 25 ?
> similar to ftp on 21 the initial connection is on 25 and then does all
communication happen on this port ?



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