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I am trying to experiment with some p2p networking. Upon doing some research, one of the biggest obstacle I learnt is "What if a client is behind a NAT/Firewall", later on I discovered about Hole Punching but that it is not always guaranteed to work.
As far a I understand, I don't understand why it might fail, This is what I know so far:
Based on the diagram above, this is how I understand how a successful connection can be established.
Alice joins the network (1) by creating connection to a directory-server. When this happens, Alice's NAT creates a mapping from her public ip to her local ip.
The directory server receives the connection and store Alice's public ip:port in the directory
Bob does the same (2), Joins the network and publishes his ip:port in the directory
Alice wants to communicate with bob. So she looks up Bob's ip:port from the directory. (3)
Alice sends data on Bob's ip:port which she got from the server. (5)
Since Bob also has a mapping from is ip:port to his local ip:port, the NAT simply forwards any data received on Bob's public ip:port to his computer.
Same works for Alice
I hope I was clear in my explanation of what I understand. My question is, what is so hard or unreliable about this? i must be clearly missing something. Can you explain me what it is?
One problem is that the NAT mappings in Alice's NAT server may time out, either after a fixed time, or after a period of inactivity.
A second potential problem is that the NAT server could make the restriction that Alice's NAT mapping is only "good" for TCP connections established by Alice, or connections between Alice and the initial IP "she" connected to. (In other words, direct communication between Alice & Bob may be blocked.)
And so on.
The problem is that the behaviour of a NAT server is highly dependent on how the managing organization's configuration / policy decisions. Many of these decisions could mean that your particular P2P usage pattern won't work reliably ... or at all.
So then is my whole idea about hole punching wrong?
No. It just means that it won't always work.
Possibly the biggest problem in NAT holepunching is lack of port consistency. For your implementation to work, at least one of the two NATs must support it.
Port consistency is where the same (local ip, local port) is mapped to the same (external ip, external port) regardless of the target (destination ip, destination port). Without this, the port seen by the directory server is not helpful to the client since it will not be the same port the clients will need to talk to each other.
(Note that this is a weaker requirement than port preservation, where external port == local port.)
Unfortunately for P2P communication, most NATs are some flavor of Symmetric NAT and do not have consistent port mappings.
Firewalls are typically stateful. Bob (2) establishing communications with the outside directory server sets up a rule in his NAT server that allows Bob and the directory server to communicate. When the NAT server sees packets from Alice, it rejects/drops them because it hasn't seen Bob establish communications with Alice.
First of all there are 2 types of hole punching
1.UDP hole punching
2.TCP hole punching
UDP hole punching success rate is 82%
TCP hole punching success rate is 64%
I have done many UDP hole punching experiments and they were mostly all successful but not same in the case of TCP hole punching.
The reason behind the failure of TCP hole punching is only the router NAT table. I will try to explain my best:
Client 1 --> connect(client2) --Internet-- connect(client1)<-- Client 2
Now if Client1 **SYN Packet**** reaches to the client2 and **client2 **SYN packet wasn't released** , the ROUTER of client2 can do 2 things:
1. send RST packet back as connection refused to client1.
2. drop packet immediately and no reply send to client1.
If this happens no connection will be established.
I can only suggest a solution that time difference between connect call from both the client should be very less. The connect call difference should be in milli-seconds
TIP: if you are in local network , put disable your firewall
for ubuntu user : sudo ufw disable
I think understanding how the Hole Punching really works would assist to get into why it may fail.
It was first explored by Dan Kagel, read here. In this technique, both peers are generally
assumed to be behind two different NATs. Both peers must be connected to an
intermediate server called a Rendezvous/Signaling server; there are many well-known Rendezvous protocols and SIP (RFC 3261) is the most famous one. As they are
connected to the server, they get to know about each other’s public transport
addresses through it. The public transport addresses are allocated by the NATs
in front of them. The Hole Punching process in short:
Peer 1 and Peer 2 first discover their Public Transport Addresses using
STUN Bind Request as described in RFC 8589.
Using a signaling/messaging mechanism, they exchange their Public Transport Addresses. SDP(Session Description Protocol)[16] may be used to
complete this. The Public Transport Address of Peer 1 and Peer 2 will be
assigned by NAT 1 and NAT 2 respectively.
Then both peers attempt to connect to each other using the received Public
Transport addresses. With most NATs, the first message will be dropped
by the NAT except for Full Cone NAT. But the subsequent packets will
penetrate the NAT successfully as by this time the NAT will have a mapping.
NAT can be of any type. If the NAT is, let's say, Symmetric NAT RFC 8489, Hole Punching won’t be possible. Because only an external host that receives a packet from an internal host can send a packet back. If this is the case, then the only possible way is Relaying.
Learn more about the current state of P2P communication: read RFC 5128.
This was a question raised by one of the software engineers in my organisation. I'm interested in the broadest definition.
Summary
A TCP socket is an endpoint instance defined by an IP address and a port in the context of either a particular TCP connection or the listening state.
A port is a virtualisation identifier defining a service endpoint (as distinct from a service instance endpoint aka session identifier).
A TCP socket is not a connection, it is the endpoint of a specific connection.
There can be concurrent connections to a service endpoint, because a connection is identified by both its local and remote endpoints, allowing traffic to be routed to a specific service instance.
There can only be one listener socket for a given address/port combination.
Exposition
This was an interesting question that forced me to re-examine a number of things I thought I knew inside out. You'd think a name like "socket" would be self-explanatory: it was obviously chosen to evoke imagery of the endpoint into which you plug a network cable, there being strong functional parallels. Nevertheless, in network parlance the word "socket" carries so much baggage that a careful re-examination is necessary.
In the broadest possible sense, a port is a point of ingress or egress. Although not used in a networking context, the French word porte literally means door or gateway, further emphasising the fact that ports are transportation endpoints whether you ship data or big steel containers.
For the purpose of this discussion I will limit consideration to the context of TCP-IP networks. The OSI model is all very well but has never been completely implemented, much less widely deployed in high-traffic high-stress conditions.
The combination of an IP address and a port is strictly known as an endpoint and is sometimes called a socket. This usage originates with RFC793, the original TCP specification.
A TCP connection is defined by two endpoints aka sockets.
An endpoint (socket) is defined by the combination of a network address and a port identifier. Note that address/port does not completely identify a socket (more on this later).
The purpose of ports is to differentiate multiple endpoints on a given network address. You could say that a port is a virtualised endpoint. This virtualisation makes multiple concurrent connections on a single network interface possible.
It is the socket pair (the 4-tuple
consisting of the client IP address,
client port number, server IP address,
and server port number) that specifies
the two endpoints that uniquely
identifies each TCP connection in an
internet. (TCP-IP Illustrated Volume 1, W. Richard Stevens)
In most C-derived languages, TCP connections are established and manipulated using methods on an instance of a Socket class. Although it is common to operate on a higher level of abstraction, typically an instance of a NetworkStream class, this generally exposes a reference to a socket object. To the coder this socket object appears to represent the connection because the connection is created and manipulated using methods of the socket object.
In C#, to establish a TCP connection (to an existing listener) first you create a TcpClient. If you don't specify an endpoint to the TcpClient constructor it uses defaults - one way or another the local endpoint is defined. Then you invoke the Connect
method on the instance you've created. This method requires a parameter describing the other endpoint.
All this is a bit confusing and leads you to believe that a socket is a connection, which is bollocks. I was labouring under this misapprehension until Richard Dorman asked the question.
Having done a lot of reading and thinking, I'm now convinced that it would make a lot more sense to have a class TcpConnection with a constructor that takes two arguments, LocalEndpoint and RemoteEndpoint. You could probably support a single argument RemoteEndpoint when defaults are acceptable for the local endpoint. This is ambiguous on multihomed computers, but the ambiguity can be resolved using the routing table by selecting the interface with the shortest route to the remote endpoint.
Clarity would be enhanced in other respects, too. A socket is not identified by the combination of IP address and port:
[...]TCP demultiplexes incoming segments using all four values that comprise the local and foreign addresses: destination IP address, destination port number, source IP address, and source port number. TCP cannot determine which process gets an incoming segment by looking at the destination port only. Also, the only one of the [various] endpoints at [a given port number] that will receive incoming connection requests is the one in the listen state. (p255, TCP-IP Illustrated Volume 1, W. Richard Stevens)
As you can see, it is not just possible but quite likely for a network service to have numerous sockets with the same address/port, but only one listener socket on a particular address/port combination. Typical library implementations present a socket class, an instance of which is used to create and manage a connection. This is extremely unfortunate, since it causes confusion and has lead to widespread conflation of the two concepts.
Hagrawal doesn't believe me (see comments) so here's a real sample. I connected a web browser to http://dilbert.com and then ran netstat -an -p tcp. The last six lines of the output contain two examples of the fact that address and port are not enough to uniquely identify a socket. There are two distinct connections between 192.168.1.3 (my workstation) and 54.252.94.236:80 (the remote HTTP server)
TCP 192.168.1.3:63240 54.252.94.236:80 SYN_SENT
TCP 192.168.1.3:63241 54.252.94.236:80 SYN_SENT
TCP 192.168.1.3:63242 207.38.110.62:80 SYN_SENT
TCP 192.168.1.3:63243 207.38.110.62:80 SYN_SENT
TCP 192.168.1.3:64161 65.54.225.168:443 ESTABLISHED
Since a socket is the endpoint of a connection, there are two sockets with the address/port combination 207.38.110.62:80 and two more with the address/port combination 54.252.94.236:80.
I think Hagrawal's misunderstanding arises from my very careful use of the word "identifies". I mean "completely, unambiguously and uniquely identifies". In the above sample there are two endpoints with the address/port combination 54.252.94.236:80. If all you have is address and port, you don't have enough information to tell these sockets apart. It's not enough information to identify a socket.
Addendum
Paragraph two of section 2.7 of RFC793 says
A connection is fully specified by the pair of sockets at the ends. A
local socket may participate in many connections to different foreign
sockets.
This definition of socket is not helpful from a programming perspective because it is not the same as a socket object, which is the endpoint of a particular connection. To a programmer, and most of this question's audience are programmers, this is a vital functional difference.
#plugwash makes a salient observation.
The fundamental problem is that the TCP RFC definition of socket is in conflict with the defintion of socket used by all major operating systems and libraries.
By definition the RFC is correct. When a library misuses terminology, this does not supersede the RFC. Instead, it imposes a burden of responsibility on users of that library to understand both interpretations and to be careful with words and context. Where RFCs do not agree, the most recent and most directly applicable RFC takes precedence.
References
TCP-IP Illustrated Volume 1 The Protocols, W. Richard Stevens, 1994 Addison Wesley
RFC793, Information Sciences Institute, University of Southern California for DARPA
RFC147, The Definition of a Socket, Joel M. Winett, Lincoln Laboratory
A socket consists of three things:
An IP address
A transport protocol
A port number
A port is a number between 1 and 65535 inclusive that signifies a logical gate in a device.
Every connection between a client and server requires a unique socket.
For example:
1030 is a port.
(10.1.1.2 , TCP , port 1030) is a socket.
A socket represents a single connection between two network applications. These two applications nominally run on different computers, but sockets can also be used for interprocess communication on a single computer. Applications can create multiple sockets for communicating with each other. Sockets are bidirectional, meaning that either side of the connection is capable of both sending and receiving data.
Therefore a socket can be created theoretically at any level of the OSI model from 2 upwards. Programmers often use sockets in network programming, albeit indirectly. Programming libraries like Winsock hide many of the low-level details of socket programming. Sockets have been in widespread use since the early 1980s.
A port represents an endpoint or "channel" for network communications. Port numbers allow different applications on the same computer to utilize network resources without interfering with each other. Port numbers most commonly appear in network programming, particularly socket programming. Sometimes, though, port numbers are made visible to the casual user. For example, some Web sites a person visits on the Internet use a URL like the following:
http://www.mairie-metz.fr:8080/ In this example, the number 8080 refers to the port number used by the Web browser to connect to the Web server. Normally, a Web site uses port number 80 and this number need not be included with the URL (although it can be).
In IP networking, port numbers can theoretically range from 0 to 65535. Most popular network applications, though, use port numbers at the low end of the range (such as 80 for HTTP).
Note: The term port also refers to several other aspects of network technology. A port can refer to a physical connection point for peripheral devices such as serial, parallel, and USB ports. The term port also refers to certain Ethernet connection points, such as those on a hub, switch, or router.
ref http://compnetworking.about.com/od/basicnetworkingconcepts/l/bldef_port.htm
ref http://compnetworking.about.com/od/itinformationtechnology/l/bldef_socket.htm
With some analogy
Although a lot technical stuff is already given above for sockets...
I would like to add my answer, just in case , if somebody still could not feel the difference between ip, port and sockets
Consider a server S,
and say person X,Y,Z need a service (say chat service) from that server S
then
IP address tells --> who? is that chat server 'S' that X,Y,Z want to contact
okay, you got "who is the server"
but suppose that server 'S' is providing some other services to other people as well,say 'S' provides storage services to person A,B,C
then
port tells ---> which? service you (X,Y,Z) need i.e. chat service and not that storage service
okay.., you make server to come to know that 'chat service' is what you want and not the storage
but
you are three and the server might want to identify all the three differently
there comes the socket
now socket tells--> which one? particular connection
that is , say ,
socket 1 for person X
socket 2 for person Y
and socket 3 for person Z
Firsty, I think we should start with a little understanding of what constitutes getting a packet from A to B.
A common definition for a network is the use of the OSI Model which separates a network out into a number of layers according to purpose. There are a few important ones, which we'll cover here:
The data link layer. This layer is responsible for getting packets of data from one network device to another and is just above the layer that actually does the transmitting. It talks about MAC addresses and knows how to find hosts based on their MAC (hardware) address, but nothing more.
The network layer is the layer that allows you to transport data across machines and over physical boundaries, such as physical devices. The network layer must essentially support an additional address based mechanism which relates somehow to the physical address; enter the Internet Protocol (IPv4). An IP address can get your packet from A to B over the internet, but knows nothing about how to traverse individual hops. This is handled by the layer above in accordance with routing information.
The transport layer. This layer is responsible for defining the way information gets from A to B and any restrictions, checks or errors on that behaviour. For example, TCP adds additional information to a packet such that it is possible to deduce if packets have been lost.
TCP contains, amongst other things, the concept of ports. These are effectively different data endpoints on the same IP address to which an Internet Socket (AF_INET) can bind.
As it happens, so too does UDP, and other transport layer protocols. They don't technically need to feature ports, but these ports do provide a way for multiple applications in the layers above to use the same computer to receive (and indeed make) outgoing connections.
Which brings us to the anatomy of a TCP or UDP connection. Each features a source port and address, and a target port and address. This is so that in any given session, the target application can respond, as well as receive, from the source.
So ports are essentially a specification-mandated way of allowing multiple concurrent connections sharing the same address.
Now, we need to take a look at how you communicate from an application point of view to the outside world. To do this, you need to kindly ask your operating system and since most OSes support the Berkeley Sockets way of doing things, we see we can create sockets involving ports from an application like this:
int fd = socket(AF_INET, SOCK_STREAM, 0); // tcp socket
int fd = socket(AF_INET, SOCK_DGRAM, 0); // udp socket
// later we bind...
Great! So in the sockaddr structures, we'll specify our port and bam! Job done! Well, almost, except:
int fd = socket(AF_UNIX, SOCK_STREAM, 0);
is also possible. Urgh, that's thrown a spanner in the works!
Ok, well actually it hasn't. All we need to do is come up with some appropriate definitions:
An internet socket is the combination of an IP address, a protocol and its associated port number on which a service may provide data. So tcp port 80, stackoverflow.com is an internet socket.
An unix socket is an IPC endpoint represented in the file system, e.g. /var/run/database.sock.
A socket API is a method of requesting an application be able to read and write data to a socket.
Voila! That tidies things up. So in our scheme then,
A port is a numeric identifier which, as part of a transport layer protocol, identifies the service number which should respond to the given request.
So really a port is a subset of the requirements for forming an internet socket. Unfortunately, it just so happens that the meaning of the word socket has been applied to several different ideas. So I heartily advise you name your next project socket, just to add to the confusion ;)
Generally, you will get a lot of theoretical but one of the easiest ways to differentiate these two concepts is as follows:
In order to get a service, you need a service number. This service number is called a port. Simple as that.
For example, the HTTP as a service is running on port 80.
Now, many people can request the service, and a connection from client-server gets established. There will be a lot of connections. Each connection represents a client. In order to maintain each connection, the server creates a socket per connection to maintain its client.
A socket = IP Address + a port (numeric address)
Together they identify an end-point for a network connection on a machine. (Did I just flunk network 101?)
These are basic networking concepts so I will explain them in an easy yet a comprehensive way to understand in details.
A socket is like a telephone (i.e. end to end device for communication)
IP is like your telephone number (i.e. address for your socket)
Port is like the person you want to talk to (i.e. the service you want to order from that address)
A socket can be a client or a server socket (i.e. in a company the telephone of the customer support is a server but a telephone in your home is mostly a client)
So a socket in networking is a virtual communication device bound to a pair (ip , port) = (address , service).
Note:
A machine, a computer, a host, a mobile, or a PC can have multiple addresses , multiple open ports, and thus multiple sockets. Like in an office you can have multiple telephones with multiple telephone numbers and multiple people to talk to.
Existence of an open/active port necessitate that you must have a socket bound to it, because it is the socket that makes the port accessible. However, you may have unused ports for the time being.
Also note, in a server socket you can bind it to (a port, a specific address of a machine) or to (a port, all addresses of a machine) as in the telephone you may connect many telephone lines (telephone numbers) to a telephone or one specific telephone line to a telephone and still you can reach a person through all these telephone lines or through a specific telephone line.
You can not associate (bind) a socket with two ports as in the telephone usually you can not always have two people using the same telephone at the same time .
Advanced: on the same machine you cannot have two sockets with same type (client, or server) and same port and ip. However, if you are a client you can open two connections, with two sockets, to a server because the local port in each of these client's sockets is different)
Hope it clears you doubts
There seems to be a lot of answers equating socket with the connection between 2 PC's..which I think is absolutely incorrect. A socket has always been the endpoint on 1 PC, that may or may not be connected - surely we've all used listener or UDP sockets* at some point. The important part is that it's addressable and active. Sending a message to 1.1.1.1:1234 is not likely to work, as there is no socket defined for that endpoint.
Sockets are protocol specific - so the implementation of uniqueness that both TCP/IP and UDP/IP uses* (ipaddress:port), is different than eg., IPX (Network, Node, and...ahem, socket - but a different socket than is meant by the general "socket" term. IPX socket numbers are equivalent to IP ports). But, they all offer a unique addressable endpoint.
Since IP has become the dominant protocol, a port (in networking terms) has become synonomous with either a UDP or TCP port number - which is a portion of the socket address.
UDP is connection-less - meaning no virtual circuit between the 2 endpoints is ever created. However, we still refer to UDP sockets as the endpoint. The API functions make it clear that both are just different type of sockets - SOCK_DGRAM is UDP (just sending a message) and SOCK_STREAM is TCP (creating a virtual circuit).
Technically, the IP header holds the IP Address, and the protocol on top of IP (UDP or TCP) holds the port number. This makes it possible to have other protocols (eg. ICMP that have no port numbers, but do have IP addressing information).
Short brief answer.
A port can be described as an internal address within a host that identifies a program or process.
A socket can be described as a programming interface allowing a program to communicate with other programs or processes, on the internet, or locally.
They are terms from two different domains: 'port' is a concept from TCP/IP networking, 'socket' is an API (programming) thing. A 'socket' is made (in code) by taking a port and a hostname or network adapter and combining them into a data structure that you can use to send or receive data.
After reading the excellent up-voted answers, I found that the following point needed emphasis for me, a newcomer to network programming:
TCP-IP connections are bi-directional pathways connecting one address:port combination with another address:port combination. Therefore, whenever you open a connection from your local machine to a port on a remote server (say www.google.com:80), you are also associating a new port number on your machine with the connection, to allow the server to send things back to you, (e.g. 127.0.0.1:65234). It can be helpful to use netstat to look at your machine's connections:
> netstat -nWp tcp (on OS X)
Active Internet connections
Proto Recv-Q Send-Q Local Address Foreign Address (state)
tcp4 0 0 192.168.0.6.49871 17.172.232.57.5223 ESTABLISHED
...
A socket is a communication endpoint. A socket is not directly related to the TCP/IP protocol family, it can be used with any protocol your system supports. The C socket API expects you to first get a blank socket object from the system that you can then either bind to a local socket address (to directly retrieve incoming traffic for connection-less protocols or to accept incoming connection requests for connection-oriented protocols) or that you can connect to a remote socket address (for either kind of protocol). You can even do both if you want to control both, the local socket address a socket is bound to and the remote socket address a socket is connected to. For connection-less protocols connecting a socket is even optional but if you don't do that, you'll have to also pass the destination address with every packet you want to send over the socket as how else would the socket know where to send this data to? Advantage is that you can use a single socket to send packets to different socket addresses. Once you have your socket configured and maybe even connected, consider it to be a bi-directional communication pipe. You can use it to pass data to some destination and some destination can use it to pass data back to you. What you write to a socket is send out and what has been received is available for reading.
Ports on the other hand are something that only certain protocols of the TCP/IP protocol stack have. TCP and UDP packets have ports. A port is just a simple number. The combination of source port and destination port identify a communication channel between two hosts. E.g. you may have a server that shall be both, a simple HTTP server and a simple FTP server. If now a packet arrives for the address of that server, how would it know if that is a packet for the HTTP or the FTP server? Well, it will know so as the HTTP server will run on port 80 and the FTP server on port 21, so if the packet arrives with a destination port 80, it is for the HTTP server and not for the FTP server. Also the packet has a source port since without such a source port, a server could only have one connection to one IP address at a time. The source port makes it possible for a server to distinguish otherwise identical connections: they all have the same destination port, e.g. port 80, the same destination IP (the IP of the server), and the same source IP, as they all come from the same client, but as they have different source ports, the server can distinguish them from each other. And when the server sends back replies, it will do so to the port the request came from, that way the client can also distinguish different replies it receives from the same server.
A socket is a special type of file handle which is used by a process to request network services from the operating system.
A socket address is the triple:
{protocol, local-address, local-process} where the local process is identified by a port number.
In the TCP/IP suite, for example:
{tcp, 193.44.234.3, 12345}
A conversation is the communication link between two processes thus depicting an association between two.
An association is the 5-tuple that completely specifies the two processes that comprise a connection:
{protocol, local-address, local-process, foreign-address, foreign-process}
In the TCP/IP suite, for example:
{tcp, 193.44.234.3, 1500, 193.44.234.5, 21}
could be a valid association.
A half-association is either:
{protocol, local-address, local-process}
or
{protocol, foreign-address, foreign-process}
which specify each half of a connection.
The half-association is also called a socket or a transport address. That is, a socket is an end point for communication that can be named and addressed in a network.
The socket interface is one of several application programming interfaces (APIs) to the communication protocols. Designed to be a generic communication programming interface, it was first introduced by the 4.2BSD UNIX system. Although it has not been standardized, it has become a de facto industry standard.
A socket address is an IP address & port number
123.132.213.231 # IP address
:1234 # port number
123.132.213.231:1234 # socket address
A connection occurs when 2 sockets are bound together.
An application consists of pair of processes which communicate over the network (client-server pair). These processes send and receive messages, into and from the network through a software interface called socket. Considering the analogy presented in the book "Computer Networking: Top Down Approach". There is a house that wants to communicate with other house. Here, house is analogous to a process, and door to a socket. Sending process assumes that there is a infrastructure on the other side of the door that will transport the data to the destination. Once the message is arrived on the other side, it passes through receiver's door (socket) into the house (process). This illustration from the same book can help you:
Sockets are part of transport layer, which provides logical communication to applications. This means that from application's point of view both hosts are directly connected to each other, even though there are numerous routers and/or switches between them. Thus a socket is not a connection itself, it's the end point of the connection. Transport layer protocols are implemented only on hosts, and not on intermediate routers.
Ports provide means of internal addressing to a machine. The primary purpose it to allow multiple processes to send and receive data over the network without interfering with other processes (their data). All sockets are provided with a port number. When a segment arrives to a host, the transport layer examines the destination port number of the segment. It then forwards the segment to the corresponding socket. This job of delivering the data in a transport layer segment to the correct socket is called de-multiplexing. The segment's data is then forwarded to the process attached to the socket.
The port was the easiest part, it is just a unique identifier for a socket. A socket is something processes can use to establish connections and to communicate with each other. Tall Jeff had a great telephone analogy which was not perfect, so I decided to fix it:
ip and port ~ phone number
socket ~ phone device
connection ~ phone call
establishing connection ~ calling a number
processes, remote applications ~ people
messages ~ speech
A socket is a structure in your software. It's more-or-less a file; it has operations like read and write. It isn't a physical thing; it's a way for your software to refer to physical things.
A port is a device-like thing. Each host has one or more networks (those are physical); a host has an address on each network. Each address can have thousands of ports.
One socket only may be using a port at an address. The socket allocates the port approximately like allocating a device for file system I/O. Once the port is allocated, no other socket can connect to that port. The port will be freed when the socket is closed.
Take a look at TCP/IP Terminology.
from Oracle Java Tutorial:
A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to.
Port and socket can be compared to the Bank Branch.
The building number of the "Bank" is analogous to IP address.
A bank has got different sections like:
Savings account department
Personal loan department
Home loan department
Grievance department
So 1 (savings account department), 2 (personal loan department), 3 (home loan department) and 4 (grievance department) are ports.
Now let us say you go to open a savings account, you go to the bank (IP address), then you go to "savings account department" (port number 1), then you meet one of the employees working under "savings account department". Let us call him SAVINGACCOUNT_EMPLOYEE1 for opening account.
SAVINGACCOUNT_EMPLOYEE1 is your socket descriptor, so there may be
SAVINGACCOUNT_EMPLOYEE1 to SAVINGACCOUNT_EMPLOYEEN. These are all socket descriptors.
Likewise, other departments will be having employess working under them and they are analogous to socket.
A socket is a data I/O mechanism. A port is a contractual concept of a communication protocol. A socket can exist without a port. A port can exist witout a specific socket (e.g. if several sockets are active on the same port, which may be allowed for some protocols).
A port is used to determine which socket the receiver should route the packet to, with many protocols, but it is not always required and the receiving socket selection can be done by other means - a port is entirely a tool used by the protocol handler in the network subsystem. e.g. if a protocol does not use a port, packets can go to all listening sockets or any socket.
Port:
A port can refer to a physical connection point
for peripheral devices such as serial, parallel, and USB ports.
The term port also refers to certain Ethernet connection points, s
uch as those on a hub, switch, or router.
Socket:
A socket represents a single connection between two network applications.
These two applications nominally run on different computers,
but sockets can also be used for interprocess communication on a single computer.
Applications can create multiple sockets for communicating with each other.
Sockets are bidirectional, meaning that either side of the connection is capable of both sending and receiving data.
Relative TCP/IP terminology which is what I assume is implied by the question. In layman's terms:
A PORT is like the telephone number of a particular house in a particular zip code. The ZIP code of the town could be thought of as the IP address of the town and all the houses in that town.
A SOCKET on the other hand is more like an established phone call between telephones of a pair of houses talking to each other. Those calls can be established between houses in the same town or two houses in different towns. It's that temporary established pathway between the pair of phones talking to each other that is the SOCKET.
In a broad sense,
Socket - is just that, a socket, just like your electrical, cable or telephone socket. A point where "requisite stuff" (power, signal, information) can go out and come in from. It hides a lot of detailed stuff, which is not required for the use of the "requisite stuff". In software parlance, it provides a generic way of defining a mechanism of communication between two entities (those entities could be anything - two applications, two physically separate devices, User & Kernel space within an OS, etc)
A Port is an endpoint discriminator. It differentiates one endpoint from another. At networking level, it differentiates one application from another, so that the networking stack can pass on information to the appropriate application.
Socket is an abstraction provided by kernel to user applications for data I/O. A socket type is defined by the protocol it's handling, an IPC communication etc. So if somebody creates a TCP socket he can do manipulations like reading data to socket and writing data to it by simple methods and the lower level protocol handling like TCP conversions and forwarding packets to lower level network protocols is done by the particular socket implementation in the kernel. The advantage is that user need not worry about handling protocol specific nitigrities and should just read and write data to socket like a normal buffer. Same is true in case of IPC, user just reads and writes data to socket and kernel handles all lower level details based on the type of socket created.
Port together with IP is like providing an address to the socket, though its not necessary, but it helps in network communications.
A port denotes a communication endpoint in the TCP and UDP transports for the IP network protocol. A socket is a software abstraction for a communication endpoint commonly used in implementations of these protocols (socket API). An alternative implementation is the XTI/TLI API.
See also:
Stevens, W. R. 1998, UNIX Network Programming: Networking APIs: Sockets and XTI; Volume 1, Prentice Hall.
Stevens, W. R., 1994, TCP/IP Illustrated, Volume 1: The Protocols, Addison-Wesley.
Socket is SW abstraction of networking endpoint, used as the interface to the application. In Java, C# it is represented by object, in Linux, Unix it is a file.
Port is just a property of a socket you have specify if you want to establish a communication. To receieve packet from a socket you have to bind it to specific local port and NIC (with local IP address) or all NICs (INADDR_ANY is specified in the bind call). To send packet, you have to specify port and IP of the remote socket.
Already theoretical answers have been given to this question. I would like to give a practical example to this question, which will clear your understanding about Socket and Port.
I found it here
This example will walk you thru the process of connecting to a website, such as Wiley. You would open your web browser (like Mozilla Firefox) and type www.wiley.com into the address bar. Your web browser uses a Domain Name System (DNS) server to look up the name www.wiley.com to identify its IP address is. For this example, the address is 192.0.2.100.
Firefox makes a connection to the 192.0.2.100 address and to the port
where the application layer web server is operating. Firefox knows
what port to expect because it is a well-known port . The well-known
port for a web server is TCP port 80.
The destination socket that Firefox attempts to connect is written as
socket:port, or in this example, 192.0.2.100:80. This is the server
side of the connect, but the server needs to know where to send the
web page you want to view in Mozilla Firefox, so you have a socket for
the client side of the connection also.
The client side connection is made up of your IP address, such as
192.168.1.25, and a randomly chosen dynamic port number. The socket associated with Firefox looks like 192.168.1.25:49175. Because web
servers operate on TCP port 80, both of these sockets are TCP sockets,
whereas if you were connecting to a server operating on a UDP port,
both the server and client sockets would be UDP sockets.
A single port can have one or more sockets connected with different external IP's like a multiple electrical outlet.
TCP 192.168.100.2:9001 155.94.246.179:39255 ESTABLISHED 1312
TCP 192.168.100.2:9001 171.25.193.9:61832 ESTABLISHED 1312
TCP 192.168.100.2:9001 178.62.199.226:37912 ESTABLISHED 1312
TCP 192.168.100.2:9001 188.193.64.150:40900 ESTABLISHED 1312
TCP 192.168.100.2:9001 198.23.194.149:43970 ESTABLISHED 1312
TCP 192.168.100.2:9001 198.49.73.11:38842 ESTABLISHED 1312
A socket is basically an endpoint for network communication, consisting of at least an IP-address and a port. In Java/C# a socket is a higher level implementation of one side of a two-way connection.
Also, a (non-normative) definition in the Java Tutorial.