The differentiating factor is source port.
A socket is made of four parts
Source IP, Source Port, Destination IP, Destination port
so this combination needs to be unique, rather than any particular part.
This socket is a feature of UDP and TCP so they can be handled this way.
This also shows the problems of proxying ICMP services although some ICMP
services allow an identifier field that lets a firewall keep track of
ingoing versus outgoing packets (echo and echo response).

To:
firewallslists.gnac.net
cc:
(bcc: Bill Royds/HullOttawa/PCH/CA)

Subject:
Multiple Clients

Hi All experts,

I wanna know the mechanism by which firewall
differentiate multiple clients under same IP. Let
consider the scenario where multiple PCs are connected
to the net via the same firewall. All PCs use the same
IP proxy to access the internet.

Let's say a few users are voice chatting on internet.
All are using same listening port for RTP. Here what I
am confused about is how does a firewall or proxy
differentiate between each client, since packets will
come on the same port and same IP for all internal
users. Does firewall/proxy multicast all the packets
that arrive onto that particular listen port in its
LAN ? Or is it a different mechanism ?

I will highly appreciate your reply.

Thanks in advance,
Ram Prasad

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