[Gllug] sorts of ports

[David Damerell]

On Saturday, 1 Sep 2001, Leigh Mason wrote:
>i'm having trouble visualising what's happening when people talk about
>entering a machine via port x or port y, with regard to network
>communication.
>i understand that any i/o device has to have  interface circuits to
>communicate data transfer to the cpu.
>and that within each interface there are registers similar to the cpu and at
>least one of those registers is a buffer register (for data) called a port.
>(please correct me if i'm wrong).
>when a connection is made between two computers there is only one interface
>in action (modem for example)  that the data is being carried across. the
>interface has a 16 bit data register allowing the 65535 possible port
>numbers, but it is still just one physical port - right?

Er, not quite.

Normally your Ethernet (or modem) hardware knows nothing about TCP/IP
at all, including IP port numbers. IP packets are entirely
encapsulated in packets for the lower-level transport, like this;

/here's the stuff your Ethernet /this here is an IP packet   \\
|card understands, that tells   |that the Ethernet card will ||
|it this is for this machine and|pass unchanged to the OS and||
\not a lot else.                \contains an IP port number  //

The OS's IP stack has an idea of what ports are open and belong to
which processes, and passes the IP packet to the appropriate process.

-- 
David Damerell <damerell at chiark.greenend.org.uk> flcl?

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