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Introduction to networking

Networks consist of two or more devices, such as computer systems, printers, and related equipment, which are connected by either physical cabling or wireless links for the purpose of sharing and distributing information among the connected devices.

This section provides general and specific information pertaining to networking, including an overview of network concepts and detailed discussion of popular network protocols.

The Transmission Control Protocol and Internet Protocol (TCP/IP)

The Transmission Control Protocol and Internet Protocol is a standard set of protocols developed in the late 1970s by the Defense Advanced Research Projects Agency (DARPA) as a means of communication between different types of computers and computer networks. TCP/IP is the driving force of the Internet, and thus it is the most popular set of network protocols on Earth.

TCP/IP overview

The two protocol components of TCP/IP deal with different aspects of computer networking.

Internet Protocol – the “IP” of TCP/IP – is a connectionless protocol that deals only with network packet routing using the IP Datagram as the basic unit of networking information. The IP Datagram consists of a header followed by a message.
Transmission Control Protocol – the “TCP” of TCP/IP – enables network hosts to establish connections that may be used to exchange data streams. TCP also guarantees that data sent between connections is delivered, and that it arrives at one network host in the same order as sent from another network host.

TCP/IP configuration

The TCP/IP protocol configuration consists of several elements that must be set by editing the appropriate configuration files, or by deploying solutions such as the Dynamic Host Configuration Protocol (DHCP) server which can, in turn, be configured to provide the proper TCP/IP configuration settings to network clients automatically. These configuration values must be set correctly in order to facilitate the proper network operation of your Ubuntu system.

The common configuration elements of TCP/IP and their purposes are as follows:

IP address: The IP address is a unique identifying string expressed as four decimal numbers ranging from zero (0) to two-hundred and fifty-five (255), separated by periods, with each of the four numbers representing eight (8) bits of the address for a total length of thirty-two (32) bits for the whole address. This format is called dotted quad notation.
Netmask: The subnet mask (or simply, netmask) is a local bit mask, or set of flags, which separate the portions of an IP address significant to the network from the bits significant to the subnetwork. For example, in a Class C network, the standard netmask is 255.255.255.0 which masks the first three bytes of the IP address and allows the last byte of the IP address to remain available for specifying hosts on the subnetwork.
Network address: The network address represents the bytes comprising the network portion of an IP address. For example, the host 12.128.1.2 in a Class A network would use 12.0.0.0 as the network address, where twelve (12) represents the first byte of the IP address, (the network part) and zeroes (0) in all of the remaining three bytes to represent the potential host values. A network host using the private IP address 192.168.1.100 would in turn use a network address of 192.168.1.0, which specifies the first three bytes of the Class C 192.168.1 network and a zero (0) for all the possible hosts on the network.
Broadcast address: The broadcast address is an IP address that allows network data to be sent simultaneously to all hosts on a given subnetwork, rather than specifying a particular host. The standard general broadcast address for IP networks is 255.255.255.255, but this broadcast address cannot be used to send a broadcast message to every host on the Internet because routers block it. A more appropriate broadcast address is set to match a specific subnetwork. For example, on the private Class C IP network, 192.168.1.0, the broadcast address is 192.168.1.255. Broadcast messages are typically produced by network protocols such as the Address Resolution Protocol (ARP) and the Routing Information Protocol (RIP).
Gateway address: A gateway address is the IP address through which a particular network, or host on a network, may be reached. If one network host wishes to communicate with another network host, and that host is not located on the same network, then a gateway must be used. In many cases, the gateway address will be that of a router on the same network, which will in turn pass traffic on to other networks or hosts, such as Internet hosts. The value of the Gateway Address setting must be correct, or your system will not be able to reach any hosts beyond those on the same network.
Nameserver address: Nameserver addresses represent the IP addresses of Domain Name Service (DNS) systems, which resolve network hostnames into IP addresses. There are three levels of nameserver addresses, which may be specified in order of precedence: The primary nameserver, the secondary nameserver, and the tertiary nameserver. So that your system can resolve network hostnames into their corresponding IP addresses, you must specify valid nameserver addresses that you are authorized to use in your system’s TCP/IP configuration. In many cases, these addresses can and will be provided by your network service provider, but many free and publicly accessible nameservers are available for use, such as the Level3 (Verizon) servers with IP addresses from 4.2.2.1 to 4.2.2.6.

Tip
The IP address, netmask, network address, broadcast address, gateway address, and nameserver addresses are typically specified via the appropriate directives in the file /etc/network/interfaces. For more information, view the system manual page for interfaces, with the following command typed at a terminal prompt:

man interfaces

IP routing

IP routing is a way of specifying and discovering paths in a TCP/IP network that network data can be sent along. Routing uses a set of routing tables to direct the forwarding of network data packets from their source to the destination, often via many intermediary network nodes known as routers. There are two primary forms of IP routing: static routing and dynamic routing.

Static routing involves manually adding IP routes to the system’s routing table, and this is usually done by manipulating the routing table with the route command. Static routing enjoys many advantages over dynamic routing, such as simplicity of implementation on smaller networks, predictability (the routing table is always computed in advance, and thus the route is precisely the same each time it is used), and low overhead on other routers and network links due to the lack of a dynamic routing protocol. However, static routing does present some disadvantages as well. For example, static routing is limited to small networks and does not scale well. Static routing also fails completely to adapt to network outages and failures along the route due to the fixed nature of the route.

Dynamic routing depends on large networks with multiple possible IP routes from a source to a destination and makes use of special routing protocols, such as the Router Information Protocol (RIP), which handle the automatic adjustments in routing tables that make dynamic routing possible. Dynamic routing has several advantages over static routing, such as superior scalability and the ability to adapt to failures and outages along network routes. Additionally, there is less manual configuration of the routing tables, since routers learn from one another about their existence and available routes. This trait also prevents mistakes being introduced into the routing tables via human error. Dynamic routing is not perfect, however, and presents disadvantages such as heightened complexity and additional network overhead from router communications, which does not immediately benefit the end users but still consumes network bandwidth.

About TCP and UDP

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are the most common protocols used to transfer data over networks.

TCP is a connection-based protocol, offering error correction and guaranteed delivery of data via what is known as flow control. Flow control determines when the flow of a data stream needs to be stopped, and previously-sent data packets should be re-sent due to problems such as collisions, for example, thus ensuring complete and accurate delivery of the data. TCP is typically used in the exchange of important information such as database transactions.

UDP on the other hand, is a connectionless protocol which seldom deals with the transmission of important data because it lacks flow control or any other method to ensure reliable delivery of the data. UDP is commonly used in such applications as audio and video streaming, where it is considerably faster than TCP due to the lack of error correction and flow control, and where the loss of a few packets is not generally catastrophic.

Internet Control Messaging Protocol (ICMP)

The Internet Control Messaging Protocol is an extension to the Internet Protocol (IP) as defined in the Request For Comments (RFC) #792, and supports network packets containing control, error, and informational messages. ICMP is used by such network applications as the ping utility, which can determine the availability of a network host or device. Examples of some error messages returned by ICMP which are useful to both network hosts and devices such as routers, include Destination Unreachable and Time Exceeded.

About daemons

Daemons are special system applications which typically execute continuously in the background and await requests for the functions they provide from other applications. Many daemons are network-centric; that is, a large number of daemons executing in the background on an Ubuntu system may provide network-related functionality. Such network daemons include the Hyper Text Transport Protocol Daemon (httpd), which provides web server functionality; the Secure SHell Daemon (sshd), which provides secure remote login shell and file transfer capabilities; and the Internet Message Access Protocol Daemon (imapd), which provides E-Mail services.