Network Services
| Network services provide the ability to manage and administer TCP/IP-based networks. Today, it is quite likely that a network of any size will use a number of network services, making them an important component of network administration. The following sections discuss each of the network services covered on the CompTIA exam.
Domain Name Service (DNS)The function of the DNS service is to resolve hostnames, such as server1.examcram.com, to IP addresses. Such a resolution system makes it possible for people to remember the names of, and refer to frequently used hosts, using the easy-to-remember hostnames rather than the hard-to-remember IP addresses.
Similar to other TCP/IP-based services, DNS is a platform-independent protocol. Therefore, it can be used on Linux, UNIX, Windows, NetWare, and almost every other platform. On networks where there is no DNS server, it is possible to resolve hostnames to IP address using the HOSTS file; however, such environments are becoming increasingly rare. All common network operating systems now include DNS server application software. The HOSTS file is a text file, found on almost all PC operating systems, in which you can place hostname-to-IP-address resolution information. When HOSTS files are used, it's up to the administrator to manually make changes to the file if needed. This factor alone is sufficient to make the installation of a DNS server an obvious choice.
Network Address Translation (NAT) and Internet Connection Sharing (ICS)NAT and ICS are two strategies that enable networks to access the Internet through a single connection. Having a single access point for the network enables an organization to have Internet access with a single IP address. NATThe basic principle of NAT is that many computers can "hide" behind a single registered IP address or a group of registered IP addresses. Using NAT means that, in its most basic implementation, only one registered IP address is needed on the external interface of the system that is acting as the gateway between an internal private network and an external public network such as the Internet. A system performing the NAT service funnels the requests that are given to it to the external network. For instance, a client requests a website, and the request goes through the NAT server to the Internet. To the remote system, the request looks like it is originating from a single address, that of the NAT server, and not the individual client systems making the request. The system that is performing the NAT function keeps track of who asked for what and makes sure that when the data is returned, it is directed to the correct system. Servers that provide NAT functionality do so in different ways. For example, it is possible to statically map a single internal IP address to a single external one so that outgoing requests are always tagged with the same IP address. Alternatively, if you have a group of public IP addresses, you can have the NAT system assign addresses to devices on a first-come, first-serve basis. Either way, the basic function of NAT is the same. ICSAlthough ICS is discussed separately from NAT, it is nothing more than an implementation of NAT on Windows platforms since Windows Me. ICS makes it very simple to share an Internet connection with multiple systems on the network. Because ICS was intended as a simple mechanism for a small office network or a home network to share a single Internet connection, configuration is simple. However, simplicity is also the potential downfall of ICS. ICS provides no security, and the system providing the shared connection is not secure against outside attacks. For that reason, ICS should be used only when no other facilities are available or in conjunction with a firewall application, which later versions of Microsoft Windows, such as XP, now include. Windows Internet Name Service (WINS)On Windows networks, a system called WINS enables Network Basic Input/Output System (NetBIOS) names to be resolved to IP addresses. NetBIOS name resolution is necessary on Windows networks so that systems can locate and access each other by using the NetBIOS computer name rather than the IP address. It's a lot easier for a person to remember a computer called secretary than to remember its IP address, 192.168.2.34. The NetBIOS name needs to be resolved to an IP address and subsequently to a MAC address (by ARP). NetBIOS name resolution can be performed three ways on a network. The simplest way is to use a WINS server on the network that will automatically perform the NetBIOS name resolution. If a WINS server is not available, the NetBIOS name resolution can be performed statically using a LMHOSTS file. Using a LMHOSTS file requires that you manually configure at least one text file with the entries. As you can imagine, this can be a time-consuming process, particularly if the systems on the network change frequently. The third method, and the default, is that systems will resolve NetBIOS names using broadcasts. There are two problems with this approach. First, the broadcasts create additional network traffic, and second, the broadcasts cannot traverse routers unless the router is configured to forward them. This means that resolutions between network segments are not possible. Simple Network Management Protocol (SNMP)SNMP is a management protocol that enables network devices to communicate information about their state to a central system. It also enables the central system to pass configuration parameters to the devices. In an SNMP configuration, a system known as a manager acts as the central communication point for all the SNMP-enabled devices on the network. On each device that is to be managed and monitored via SNMP, software called an SNMP agent is set up and configured with the IP address of the manager. Depending on the configuration, the SNMP manager is then capable of communicating with and retrieving information from the devices running the SNMP agent software. In addition, the agent is able to communicate the occurrence of certain events to the SNMP manager as they happen. These messages are known as traps.
An important part of SNMP is an SNMP management system, which is a computer running a special piece of software called a Network Management System (NMS). These software applications can be free, or they can cost thousands of dollars. The difference between the free applications and those that cost a great deal of money normally boils down to functionality and support. All NMS systems, regardless of cost, offer the same basic functionality. Today, most NMS applications use graphical maps of the network to locate a device and then query it. The queries are built in to the application and are triggered by a point and click. You can actually issue SNMP requests from a command-line utility, but with so many tools available, it is simply not necessary. An SNMP agent can be any device capable of running a small software component that facilitates communication with an SNMP manager. SNMP agent functionality is supported by almost any device designed to be connected to a network. Network File System (NFS)The Network File System (NFS) is a protocol and network service that allows you to access file systems on remote computers across the network. NFS is most commonly associated with UNIX and Linux operating system platforms, but versions of NFS are available for a wide range of server operating systems including Microsoft Windows. From a client perspective, UNIX and Linux implementations use NFS as the default file system access mechanism. However, versions of NFS client software are also available for most commonly deployed workstation operating systems.
Zero Configuration (Zeroconf)Zero Configuration (Zeroconf) provides a means of networking computer systems together without requiring specific network configuration. This approach is becoming increasingly necessary as we use a larger number and wider variety of computing devices in a networked scenario. There are three basic requirements for a system to support Zeroconf. First, the system must be capable of assigning itself an IP address without the need for a DHCP server. Second, the system must be capable of resolving the hostname of another system to an IP address without the use of a DNS server. Finally, a system must be capable of locating or advertising services on the network without a directory services system such as Microsoft's Active Directory or Novell Directory Services. Currently, Zero Configuration is supported, with additional software, by Mac and Windows operating systems, as well as by Linux and UNIX.
Server Message Block (SMB)Server Message Block (SMB) is an application and presentation layer protocol that provides a mechanism to access shared network resources such as files or printers on network servers. SMB is the default file access method used on Windows networks. Today, SMB is more commonly referred to as the Common Internet File System (CIFS), though the functionality remains the same. On a network that uses Windows servers and clients, administrators access the functionality of SMB through Windows Explorer and the command line NET utility.
Apple File Protocol (AFP)The Apple File Protocol (AFP), more correctly called the AppleTalk Filing Protocol, is to Apple systems what NFS is to Linux/UNIX systems, and SMB or CIFS is to Windows Systems. It is a protocol through which the file system on remote computers can be accessed. AFP is not widely used outside of Apple networks, and unless you are working on networks that use Apple Macintosh systems, you are unlikely to encounter AFP.
Line Printer Daemon (LPD)The Line Printer Daemon (LPD) protocol provides print services on both client and server systems. The most common use of LPD is as a print server and client on UNIX and Linux systems. As well as providing the basic print mechanisms, LPD supports a set of commands that enable the print queue to be controlled. It also provides commands for controlling print jobs once they have been placed in the print queue. TCP/IP Service SummaryTable 5.8 helps you quickly identify the purpose and function of each of the TCP/IP services covered in the previous sections.
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