6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM 85;:+ The route command is used to configure network routing tables. This may be a tedious task but is sometimes necessary for reasons of security or because a specific route has to be added. The following shows the route options. C:\WINDOWS.000>route /? Manipulates network routing tables. ROUTE [-f] [command [destination] [MASK netmask] [gateway] [METRIC metric]] -f Clears the routing tables of all gateway entries. If this is used in conjunction with one of the commands, the tables are cleared prior to running the command. command Must be one of four: PRINT Prints a route ADD Adds a route DELETE Deletes a route CHANGE Modifies an existing route destination Specifies the destination host. MASK Specifies that the next parameter is the ‘netmask’ value. netmask Specifies a subnet mask value to be associated with this route entry. If not specified, it defaults to 255.255.255.255. METRIC Specifies that the next parameter ‘metric’ is the cost for this destination All symbolic names used for destination are looked up in the network database file NETWORKS. The symbolic names for gateway are looked up in the host name database file HOSTS. If the command is PRINT or DELETE, wildcards may be used for the destination and gateway, or the gateway argument may be omitted. Diagnostic notes: Invalid MASK generates an error, that is when (DEST & MASK) != DEST. Example> route ADD 255.0.0.0 157.0.0.0 MASK 155.0.0.0 157.55.80.1 The route addition failed: 87 Examples: > route PRINT > route ADD 157.0.0.0 MASK 255.0.0.0 157.55.80.1 METRIC 3 ^destination ^mask ^gateway ^metric > route PRINT > route DELETE 157.0.0.0 > route PRINT C:\WINDOWS.000> The route table exists on both hosts and routers. An individual entry is read from left to right as follows: ‘If a message is destined for network 192.100.100.0, with subnet mask 255.255.255.0, then route it through to the gateway address 192.100.100.1’. Remember that a HostID equal to 0, as used here, does not refer to a specific host but rather to the network as a whole. Routes can also be added with the route add and route delete commands. Route add 192.100.100.0 mask 255.255.255.0 192.100.100.1 will add a route and Route delete 192.100.100.0 will delete a particular route. Manual adding of routes are sometimes necessary, for example in the case where the installation of dial-up proxy :)6/6[ZOROZOKY server software on a given host sometimes overwrites the existing default gateway setting on that host in order to ‘point’ to the Internet service provider’s default gateway. This makes it impossible for the host to reach an existing adjacent network across the intermediate router, unless a manual entry is made. If said entry ‘does the job’ but disappears when the host is re-booted, the appropriate route command needs to be included in the autoexec.bat file. The following response was obtained from the route print command. Active routes: Network Address Netmask Gateway Address Interface Metric 127.0.0.0 255.0.0.0 127.0.0.1 1 27.0.0.1 1 207.194.66.96 255.255.255.224 207.194.66.100 207.194.66.100 1 207.194.66.100 255.255.255.255 127.0.0.1 127.0.0.1 1 207.194.66.255 255.255.255.255 207.194.66.100 207.194.66.100 1 224.0.0.0 224.0.0.0 207.194.66.100 207.194.66.100 1 255.255.255.255 255.255.255.255 207.194.66.100 0.0.0.0 1 C:\WINDOWS.000> :NK.59:9LORK The hosts file is used on UNIX and Windows systems to resolve the mapping of a ‘name’ (any given name) to an IP address. The following is an example of a typical Windows hosts file. This file is saved in the same directory as Windows itself as c:\windows\hosts. If a user is uncertain about the correct format of the entries, a sample file can be found at c:\windows\hosts.sam. Note that, as a matter of convenience, the hosts sample file can be edited as in the accompanying example, but it MUST then be saved as hosts only, i.e. without the. same extension. In the example, host 192.100.100.2 can simply be interrogated by typing ping john. Figure 9.6 The Hosts file (courtesy of Microsoft Corporation) 10 LAN system components Objectives When you have completed this chapter you should be able to: • Explain the basic function of each of the devices listed under 10.1 • Explain the fundamental differences between the operation and application of switches (layer 2 and 3), bridges and routers 10.1 Introduction In the design of an Ethernet system there are a number of different components that can be used. These include: • Repeaters • Media converters • Bridges • Hubs • Switches • Routers • Gateways • Print servers • Terminal servers • Remote access servers • Time servers • Thin servers The lengths of LAN segments are limited due to physical and collision domain constraints and there is often a need to increase this range. This can be achieved by means of a number of interconnecting devices, ranging from repeaters to gateways. It may also be necessary to partition an existing network into separate networks for reasons of security or traffic overload. LAN system components 175 In modern network devices the functions mentioned above are often mixed: • A shared 10BaseT hub is, in fact, a multi-port repeater • A layer II switch is essentially a multi-port bridge • Segmentable and dual-speed shared hubs make use of internal bridges • Switches can function as bridges, a two-port switch being none other than a bridge • Layer III switches function as routers These examples are not meant to confuse the reader, but serve to emphasize the fact that the functions should be understood, rather than the ‘boxes’ in which they are packaged. 10.2 Repeaters A repeater operates at the physical layer of the OSI model (layer 1) and simply retransmits incoming electrical signals. This involves amplifying and re-timing the signals received on one segment onto all other segments, without considering any possible collisions. All segments need to operate with the same media access mechanism and the repeater is unconcerned with the meaning of the individual bits in the packets. Collisions, truncated packets or electrical noise on one segment are transmitted onto all other segments. 10.2.1 Packaging Repeaters are packaged either as stand-alone units (i.e. desktop models or small cigarette package-sized units) or 19" rack-mount units. Some of these can link two segments only, while larger rack-mount modular units (called Concentrators) are used for linking multiple segments. Regardless of packaging, repeaters can be classified either as local repeaters (for linking network segments that are physically in close proximity), or as remote repeaters for linking segments that are some distance apart. Figure 10.1 Repeater application 10.2.2 Local Ethernet repeaters Several options are available: • Two-port local repeaters offer most combinations of 10Base5, 10Base2, 10BaseT and 10Base-FL such as 10Base5/10Base5, 10Base2/10Base2, 10Base5/10Base2, 10Base2/10BaseT, 10BaseT/10BaseT and 10Base- FL/10Base-FL. By using such devices (often called boosters or extenders) one can, for example, extend the distance between a computer and a 10BaseT hub by up to 100 m, or extend a 10Base-FL link between two devices (such as bridges) by up to 2 km 176 Practical TCP/IP and Ethernet Networking • Multi-port local repeaters offer several ports of the same type (e.g. 4× 10Base2 or 8× 10Base5) in one unit, often with one additional connector of a different type (e.g. 10Base2 for a 10Base5 repeater). In the case of 10BaseT the cheapest solution is to use an off-the-shelf 10BaseT shared hub, which is effectively a multi-port repeater • Multi-port local repeaters are also available as chassis-type units; i.e. as frames with common back planes and removable units. An advantage of this approach is that 10Base2, 10Base5, 10BaseT and 10Base-FL can be mixed in one unit, with an option of SNMP management for the overall unit. These are also referred to as Concentrators 10.2.3 Remote repeaters Remote repeaters, on the other hand, have to be used in pairs with one repeater connected to each network segment and a fiber-optic link between the repeaters. On the network side they typically offer 10Base5, 10Base2 and 10BaseT. On the interconnecting side the choices include ‘single pair Ethernet’, using telephone cable up to 457 m in length, or single mode/multimode optic fiber, with various connector options. With 10Base-FL (backwards compatible with the old FOIRL standard), this distance can be up to 1.6 km. In conclusion it must be emphasized that although repeaters are probably the cheapest way to extend a network, they do so without separating the collision domains, or network traffic. They simply extend the physical size of the network. All segments joined by repeaters therefore share the same bandwidth and collision domain. 10.3 Media converters Media converters are essentially repeaters, but interconnect mixed media viz. copper and fiber. An example would be 10BaseT/10Base-FL. As in the case of repeaters, they are available in single and multi-port options, and in stand-alone or chassis type configurations. The latter option often features remote management via SNMP. Figure 10.2 Media converter application Models may vary between manufacturers, but generally Ethernet media converters support: • 10 Mbps (10Base2, 10BaseT, 10Base-FL – single and multi-mode) • 100 Mbps (fast) Ethernet (100Base-TX, 100Base-FX – single and multimode) • 1000 Mbps (gigabit) Ethernet (single and multimode) An added advantage of the fast and gigabit Ethernet media converters is that they support full-duplex operation that effectively doubles the available bandwidth. . (gigabit) Ethernet (single and multimode) An added advantage of the fast and gigabit Ethernet media converters is that they support full-duplex operation that effectively doubles the available bandwidth or extend a 10Base-FL link between two devices (such as bridges) by up to 2 km 176 Practical TCP/IP and Ethernet Networking • Multi-port local repeaters offer several ports of the same type. but generally Ethernet media converters support: • 10 Mbps (10Base2, 10BaseT, 10Base-FL – single and multi-mode) • 100 Mbps (fast) Ethernet (100Base-TX, 100Base-FX – single and multimode)