If using TCP, there will be a TCP Port, If using UDP there will be a UDP port. ICMP does not use ports.
To set up a conenction using TCP, three steps are involved.
I will assume a PC doing telnet.
First the PC sends a TCP sends a syn with a destination port of 23, and a source port of its choosing - typicallyy above 2048. I'll use 2069. I will also use "host" and "PC" instead of addresses.
So we get:
TCP-Syn DA - host, SA - PC , DP - 23, SP - 2069
(da - dest address, SA Source address, DP dest port, SP Source port)
Assuming the host is happy to accept the connection, it will respond:
TCP-SynAck DA - PC, SA - Host , DP - 2069, SP - 23
When the PC gets that, it acknowledges the SynAck to complete the three way handshake:
TCP-Ack DA - host, SA - PC , DP - 23, SP - 2069
At that point we have a connection. The router probably couldnt care less, normally all it is bothered about (assuming no access lists or QoS etc is:
IP DA - host, SA - PC
IP DA - PC, SA - Host
IP DA - host, SA - PC
so it will just forward them.
If the PC pings the host, all the router will care about is:
IP DA - host, SA - PC
IP DA - PC, SA - Host
Look familiar?
Where what is carried is:
ICMP DA - host, SA - PC - ECHO REQUEST
ICMP DA - PC, SA - Host - ECHO RESPONSE
Paul.
