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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Transport Layer Protocols
           9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
                9  TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

Previous Topic/Section
TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use
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Common TCP/IP Applications and Assigned Well-Known and Registered Port Numbers
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TCP/IP Sockets and Socket Pairs: Process and Connection Identification
(Page 1 of 2)

The preceding topics have illustrated the key difference between addressing at the level of the Internet Protocol, and addressing as it is seen by application processes. To summarize, at layer three, an IP address is all that is really important for properly transmitting data between IP devices. In contrast, application protocols must be concerned with the port assigned to each instance of the application, so they can properly use TCP or UDP.

Sockets: Process Identification

What this all means is that the overall identification of an application process actually uses the combination of the IP address of the host it runs on—or the network interface over which it is talking, to be more precise—and the port number which has been assigned to it. This combined address is called a socket. Sockets are specified using the following notation:

<IP Address>:<Port Number>

So, for example, if we have a Web site running on IP address 41.199.222.3, the socket corresponding to the HTTP server for that site would be 41.199.222.3:80.

Key Concept: The overall identifier of a TCP/IP application process on a device is the combination of its IP address and port number, which is called a socket.


You will also sometimes see a socket specified using a host name instead of an IP address, like this:

<Host Name>:<Port Number>

To use this descriptor, the name must first be resolved to an IP address using DNS. For example, you might find a Web site URL like this: “http://www.thisisagreatsite.com:8080”. This tells the Web browser to first resolve the name “www.thisisagreatsite.com” to an IP address using DNS, and then send a request to that address using the non-standard server port 8080, which is occasionally used instead of port 80 since it resembles it. (See the discussion of application layer addressing using URLs for much more.)

The socket is a very fundamental concept to the operation of TCP/IP application software. In fact, it is the basis for an important TCP/IP application program interface (API) with the same name: sockets. A version of this API for Windows is called Windows Sockets or WinSock, which you may have heard of before. These APIs allow application programs to easily use TCP/IP to communicate.


Previous Topic/Section
TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use
Previous Page
Pages in Current Topic/Section
1
2
Next Page
Common TCP/IP Applications and Assigned Well-Known and Registered Port Numbers
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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