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This is an unpolished draft of the third edition of this e-book. If you find any error or have suggestions to improve the text, please create an issue via https://github.com/CNP3/ebook/issues?milestone=3 or help us by providing pull requests to close the existing issues.
 
The network and the transport layers rely on addresses that are encoded as fixed size bit strings. A network layer address uniquely identifies a host. Several transport layer entities can use the service of the same network layer. For example, a reliable transport protocol and a connectionless transport protocol can coexist on the same host. In this case, the network layer multiplexes the segments produced by these two protocols. This multiplexing is usually achieved by placing in the network packet header a field that indicates which transport protocol should process the segment. Given that there are few different transport protocols, this field does not need to be long. The port numbers play a similar role in the transport layer since they enable it to multiplex data from several application processes.
 
This is an unpolished draft of the third edition of this e-book. If you find any error or have suggestions to improve the text, please create an issue via https://github.com/CNP3/ebook/issues?milestone=2 or help us by providing pull requests to close the existing issues.
 
The reference models
 
This is an unpolished draft of the third edition of this e-book. If you find any error or have suggestions to improve the text, please create an issue via https://github.com/CNP3/ebook/issues?milestone=1 or help us by providing pull requests to close the existing issues.
 
Sharing resources
 
Applications
 
LAN/MAN Standards Committee of the IEEE Computer Society. `IEEE Standard for Information Technology - Telecommunications and information exchange between systems - local and metropolitan area networks - specific requirements - Part 11 : Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications <http://standards.ieee.org/getieee802/802.11.html>`_. IEEE, 1999.
 
A string of bits that identifies a network interface in the network layer or the datalink layer. Most addresses have a fixed length, e.g. 32 bits for :term:`IPv4`, 128 bits for :term:`IPv6` or 48 bits for :term:`Ethernet` and other related Local Area Networks.
 
The development of this edition of the textbook is carried out on `https://github.com/CNP3/ebook <https://github.com/CNP3/ebook>`_
 
Medium Access Control
 
Several `Multiple Choice Questions` and simple exercises are included in the main text and supported by the https://inginious.org platform.
 
The socket is a powerful abstraction as it allows processes to communicate even if they are located on different computers. In this specific cases, the inter-processes communication will go through a network.
 
Open questions
 
While addresses are natural for the network and transport layer entities, humans prefer to use names when interacting with network services. Names can be encoded as a character string and a mapping services allows applications to map a name into the corresponding address. Using names is friendlier for humans, but it also provides a level of indirection which is very useful in many situations.
 
In the previous section, we have explained how reliable protocols allow hosts to exchange data reliably even if the underlying physical layer is imperfect and thus unreliable. Connecting two hosts together through a wire is the first step to build a network. However, this is not sufficient. Hosts usually need to interact with other hosts that are not directly connected through a direct physical layer link. This can be achieved by adding one layer above the datalink layer: the `network` layer.
 
Given the growing complexity of computer networks, during the 1970s network researchers proposed various reference models to facilitate the description of network protocols and services. Of these, the Open Systems Interconnection (OSI) model [Zimmermann80]_ was probably the most influential. It served as the basis for the standardization work performed within the :term:`ISO` to develop global computer network standards. The reference model that we use in this book can be considered as a simplified version of the OSI reference model [#fiso-tcp]_.
 
The first step when building a network, even a worldwide network such as the Internet, is to connect two hosts together. This is illustrated in the figure below.
 
A network is designed to support a potentially large number of users that exchange information with each other. These users produce and consume information which is exchanged through the network. To support its users, a network uses several types of resources. It is important to keep in mind the different resources that are shared inside the network.
 
LAN/MAN Standards Committee of the IEEE Computer Society. IEEE Standard for Information Technology - Telecommunications and information exchange between systems - local and metropolitan area networks - specific requirements - Part 3 : Carrier Sense multiple access with collision detection (CSMA/CD) access method and physical layer specification. IEEE, 2000. Available from http://standards.ieee.org/getieee802/802.3.html