How do multiple clients connect simultaneously to one port say 80 on a server

Question

I understand the basics of how ports work  However  what I don t get is how multiple clients can simultaneously connect to say port 80  I know each client has a unique  for their machine  port  Does the server reply back from an available port to the client  and simply state the reply came from 80  How does this work

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Important   I m sorry to say that the response from  Borealid  is imprecise and somewhat incorrect - firstly there is no relation to statefulness or statelessness to answer this question  and most importantly the definition of the tuple for a socket is incorrect     First remember below two rules    Primary key of a socket  A socket is identified by  SRC-IP  SRC-PORT  DEST-IP  DEST-PORT  PROTOCOL  not by  SRC-IP  SRC-PORT  DEST-IP  DEST-PORT  - Protocol is an important part of a socket s definition  OS Process  amp  Socket mapping  A process can be associated with  can open can listen to  multiple sockets which might be obvious to many readers    Example 1  Two clients connecting to same server port means  socket1  SRC-A  100  DEST-X 80  TCP  and socket2 SRC-B  100  DEST-X 80  TCP   This means host A connects to server X s port 80 and another host B also connects to same server X to the same port 80  Now  how the server handles these two sockets depends on if the server is single threaded or multiple threaded  I ll explain this later   What is important is that one server can listen to multiple sockets simultaneously   To answer the original question of the post   Irrespective of stateful or stateless protocols  two clients can connect to same server port because for each client we can assign a different socket  as client IP will definitely differ   Same client can also have two sockets connecting to same server port - since such sockets differ by SRC-PORT  With all fairness   Borealid  essentially mentioned the same correct answer but the reference to state-less full was kind of unnecessary confusing   To answer the second part of the question on how a server knows which socket to answer  First understand that for a single server process that is listening to same port  there could be more than one sockets  may be from same client or from different clients   Now as long as a server knows which request is associated with which socket  it can always respond to appropriate client using the same socket  Thus a server never needs to open another port in its own node than the original one on which client initially tried to connect  If any server allocates different server-ports after a socket is bound  then in my opinion the server is wasting its resource and it must be needing the client to connect again to the new port assigned   A bit more for completeness   Example 2  It s a very interesting question   can two different processes on a server listen to the same port   If you do not consider protocol as one of parameter defining socket then the answer is no  This is so because we can say that in such case  a single client trying to connect to a server-port will not have any mechanism to mention which of the two listening processes the client intends to connect to  This is the same theme asserted by rule  2   However this is WRONG answer because  protocol  is also a part of the socket definition  Thus two processes in same node can listen to same port only if they are using different protocol  For example two unrelated clients  say one is using TCP and another is using UDP  can connect and communicate to the same server node and to the same port but they must be served by two different server-processes    Server Types - single  amp  multiple   When a server s processes listening to a port that means multiple sockets can simultaneously connect and communicate with the same server-process  If a server uses only a single child-process to serve all the sockets then the server is called single-process threaded and if the server uses many sub-processes to serve each socket by one sub-process then the server is called multi-process threaded server  Note that irrespective of the server s type a server can should always uses the same initial socket to respond back  no need to allocate another server-port    Suggested Books and rest of the two volumes if you can   A Note on Parent Child Process  in response to query comment of  Ioan Alexandru Cucu    Wherever I mentioned any concept in relation to two processes say A and B  consider that they are not related by parent child relationship  OS s  especially UNIX  by design allow a child process to inherit all File-descriptors  FD  from parents  Thus all the sockets  in UNIX like OS are also part of FD  that a process A listening to  can be listened by many more processes A1  A2     as long as they are related by parent-child relation to A  But an independent process B  i e  having no parent-child relation to A  cannot listen to same socket  In addition  also note that this rule of disallowing two independent processes to listen to same socket lies on an OS  or its network libraries  and by far it s obeyed by most OS s  However  one can create own OS which can very well violate this restrictions

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First off  a  port  is just a number   All a  connection to a port  really represents is a packet which has that number specified in its  destination port  header field   Now  there are two answers to your question  one for stateful protocols and one for stateless protocols   For a stateless protocol  ie UDP   there is no problem because  connections  don t exist - multiple people can send packets to the same port  and their packets will arrive in whatever sequence   Nobody is ever in the  connected  state   For a stateful protocol  like TCP   a connection is identified by a 4-tuple consisting of source and destination ports and source and destination IP addresses   So  if two different machines connect to the same port on a third machine  there are two distinct connections because the source IPs differ   If the same machine  or two behind NAT or otherwise sharing the same IP address  connects twice to a single remote end  the connections are differentiated by source port  which is generally a random high-numbered port    Simply  if I connect to the same web server twice from my client  the two connections will have different source ports from my perspective and destination ports from the web server s   So there is no ambiguity  even though both connections have the same source and destination IP addresses   Ports are a way to multiplex IP addresses so that different applications can listen on the same IP address protocol pair   Unless an application defines its own higher-level protocol  there is no way to multiplex a port   If two connections using the same protocol simultaneously have identical source and destination IPs and identical source and destination ports  they must be the same connection

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TCP   HTTP Listening On Ports  How Can Many Users Share the Same Port  So  what happens when a server listen for incoming connections on a TCP port  For example  let s say you have a web-server on port 80  Let s assume that your computer has the public IP address of 24 14 181 229 and the person that tries to connect to you has IP address 10 1 2 3  This person can connect to you by opening a TCP socket to 24 14 181 229 80  Simple enough   Intuitively  and wrongly   most people assume that it looks something like this       Local Computer      Remote Computer     --------------------------------      lt local ip gt  80        lt foreign ip gt  80         not actually what happens  but this is the conceptual model a lot of people have in mind    This is intuitive  because from the standpoint of the client  he has an IP address  and connects to a server at IP PORT  Since the client connects to port 80  then his port must be 80 too  This is a sensible thing to think  but actually not what happens  If that were to be correct  we could only serve one user per foreign IP address  Once a remote computer connects  then he would hog the port 80 to port 80 connection  and no one else could connect    Three things must be understood   1   On a server  a process is listening on a port  Once it gets a connection  it hands it off to another thread  The communication never hogs the listening port    2   Connections are uniquely identified by the OS by the following 5-tuple   local-IP  local-port  remote-IP  remote-port  protocol   If any element in the tuple is different  then this is a completely independent connection   3   When a client connects to a server  it picks a random  unused high-order source port  This way  a single client can have up to  64k connections to the server for the same destination port   So  this is really what gets created when a client connects to a server       Local Computer     Remote Computer             Role     -----------------------------------------------------------     0 0 0 0 80          lt none gt                       LISTENING     127 0 0 1 80       10 1 2 3  lt random port gt       ESTABLISHED   Looking at What Actually Happens  First  let s use netstat to see what is happening on this computer  We will use port 500 instead of 80  because a whole bunch of stuff is happening on port 80 as it is a common port  but functionally it does not make a difference        netstat -atnp   grep -i   500     As expected  the output is blank  Now let s start a web server       sudo python3 -m http server 500   Now  here is the output of running netstat again       Proto Recv-Q Send-Q Local Address           Foreign Address         State       tcp        0      0 0 0 0 0 500             0 0 0 0                 LISTEN      -    So now there is one process that is actively listening  State  LISTEN  on port 500  The local address is 0 0 0 0  which is code for  listening for all   An easy mistake to make is to listen on address 127 0 0 1  which will only accept connections from the current computer  So this is not a connection  this just means that a process requested to bind   to port IP  and that process is responsible for handling all connections to that port  This hints to the limitation that there can only be one process per computer listening on a port  there are ways to get around that using multiplexing  but this is a much more complicated topic   If a web-server is listening on port 80  it cannot share that port with other web-servers   So now  let s connect a user to our machine       quicknet -m tcp -t localhost 500 -p Test payload    This is a simple script  https   github com grokit dcore tree master apps quicknet  that opens a TCP socket  sends the payload   Test payload   in this case   waits a few seconds and disconnects  Doing netstat again while this is happening displays the following       Proto Recv-Q Send-Q Local Address           Foreign Address         State       tcp        0      0 0 0 0 0 500             0 0 0 0                 LISTEN      -     tcp        0      0 192 168 1 10 500        192 168 1 13 54240      ESTABLISHED -   If you connect with another client and do netstat again  you will see the following       Proto Recv-Q Send-Q Local Address           Foreign Address         State       tcp        0      0 0 0 0 0 500             0 0 0 0                 LISTEN      -     tcp        0      0 192 168 1 10 500        192 168 1 13 26813      ESTABLISHED -       that is  the client used another random port for the connection  So there is never confusion between the IP addresses

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Normally  for every connecting client the server forks a child process that communicates with the client  TCP   The parent server hands off to the child process an established socket that communicates back to the client   When you send the data to a socket from your child server  the TCP stack in the OS creates a packet going back to the client and sets the  from port  to 80

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Multiple clients can connect to the same port  say 80  on the server because on the server side  after creating a socket and binding  setting local IP and port  listen is called on the socket which tells the OS to accept incoming connections   When a client tries to connect to server on port 80  the accept call is invoked on the server socket  This creates a new socket for the client trying to connect and similarly new sockets will be created for subsequent clients using same port 80   Words in italics are system calls   Ref  http   www scs stanford edu 07wi-cs244b refs net2 pdf

[http] How do multiple clients connect simultaneously to one port, say 80, on a server?

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