pffff mensen, een Gateway is dus echt wel iets anders als een router. Een gateway is een soort tolk en een router alleen een dom doorgeef luik.
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Routers
In an environment that consists of several network segments with differing protocols and architectures, a bridge might be inadequate for ensuring fast communication among all segments. A network this complex needs a device that not only knows the address of each segment, but can also determine the best path for sending data and filtering broadcast traffic to the local segment. Such a device is called a "router."
Routers work at the network layer of the OSI reference model. This means they can switch and route packets across multiple networks. They do this by exchanging protocol-specific information between separate networks. Routers read complex network addressing information in the packet and, because they function at a higher layer in the OSI reference model than bridges, they have access to additional information.
Routers can provide the following functions of a bridge:
Filtering and isolating traffic
Connecting network segments
Routers have access to more of the information in packets than bridges have and use this information to improve packet deliveries. Routers are used in complex networks because they provide better traffic management. Routers can share status and routing information with one another and use this information to bypass slow or malfunctioning connections.
How Routers Work
Routers maintain their own routing tables, usually consisting of network addresses; host addresses can also be kept if the network architecture calls for it. To determine the destination address for incoming data, the routing table includes:
All known network addresses.
Instructions for connection to other networks.
The possible paths between routers.
The costs of sending data over those paths.
As shown in Figure 7.15, a router uses its data-routing table to select the best route for the data based on costs and available paths.
NOTE
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Remember that routing tables were also discussed in the context of bridges. The routing table maintained by a bridge contains MAC-sublayer addresses for each node, whereas the routing table maintained by a router contains network numbers. Although manufacturers of both types of equipment have chosen to use the term "routing table," it has a different meaning for bridges than it does for routers.
Routers require specific addresses. They understand only the network numbers that allow them to communicate with other routers and local NIC addresses. Routers do not talk to remote computers.
Figure 7.15 Routers talk to other routers, but not to remote computers
When routers receive packets destined for a remote network, they send them to the router that manages the destination network. In some ways this is an advantage because it means routers can:
Segment large networks into smaller ones.
Act as safety barriers between segments.
Prohibit broadcast storms, because broadcasts are not forwarded.
Because routers must perform complex functions on each packet, routers are slower than most bridges. As packets are passed from router to router, data-link layer source and destination addresses are stripped off and then re-created. This enables a router to route a packet from a TCP/IP Ethernet network to a server on a TCP/IP Token Ring network.
Because routers read only addressed network packets, they do not allow corrupted data to get passed onto the network. Because they do not pass corrupted data or broadcast data storms, routers put little stress on networks.
Routers do not look at the destination node address; they look only at the network address. Routers will pass information only if the network address is known. This ability to control the data passing through the router reduces the amount of traffic between networks and allows routers to use these links more efficiently than bridges.
Using the router-addressing scheme, administrators can break one large network into many separate networks, and because routers do not pass or even handle every packet, they act as a safety barrier between network segments. This can greatly reduce the amount of traffic on the network and the wait time experienced by users.
Routable Protocols Not all protocols are routable. Protocols that are routable include:
DECnet.
Internet Protocol (IP).
Internetwork Packet Exchange (IPX).
OSI.
Xerox Network System (XNS).
DDP (AppleTalk).
Protocols that are not routable include:
Local Area Transport Protocol (LAT), a protocol from Digital Equipment Corporation.
NetBEUI (NetBIOS Extended User Interface).
Routers are available that can accommodate multiple protocols such as IP and DECnet in the same network.
Choosing Paths Unlike bridges, routers can accommodate multiple active paths between LAN segments and choose among redundant paths. Because routers can link segments that use completely different data packaging and media-access schemes, there are often several paths available for the router to use. This means that if one router does not function, the data can still be passed over alternate routes.
A router can listen to a network and identify which parts are busiest. It uses this information to determine which path to send data over. If one path is very busy, the router identifies an alternative path and sends data over that one.
A router decides the path the data packet will follow by determining the number of hops between internetwork segments. Like bridges, routers build routing tables and use these in routing algorithms such as the following:
OSPF ("open shortest path first") is a link-state routing algorithm. Link-state algorithms control the routing process and allow routers to respond quickly to changes in the network.
RIP (Routing Information Protocol) uses distance-vector algorithms to determine routes. Transmission Control Protocol/Internet Protocol (TCP/IP) and IPX support RIP.
NetWare Link Services Protocol (NLSP) is a link-state algorithm to be used with IPX.
Types of Routers
The two major types of routers are:
Static.
Static routers require an administrator to manually set up and configure the routing table and to specify each route.
Dynamic.
Dynamic routers are designed to discover routes automatically and therefore require a minimal amount of setup and configuration. More sophisticated than static routers, they examine information from other routers and make packet-by-packet decisions about how to send data across the network.
Table 7.2 compares and contrasts the characteristics of static and dynamic routers.
Table 7.2 Characteristics of the Two Types of Routers
Static routers Dynamic routers
Manually set up and configure all routes. Manually configure the first route. Automatically detect additional networks and routes.
Always use the same route, determined by a routing table entry. Can choose a route based on factors such as cost and amount of link traffic.
Use a hard-coded route (designed to handle only a specific situation), not necessarily the shortest route. Can decide to send packets over alternate routes.
Are considered more secure because the administrator specifies each route. Can improve security by manually configuring the router to filter out specific network addresses and prevent traffic from going there.
Distinguishing Between Bridges and Routers
Bridges and routers can be confusing even for engineers with LAN and WAN experience because they appear to do the same things: both forward packets between networks and send data across WAN links.
A question often asked is how to decide when to use a bridge and when to use a router.
The bridge, which works at the MAC sublayer of the OSI data-link layer, sees only a node address. To be more specific, a bridge looks for a node's MAC-sublayer address in each packet. If the bridge recognizes the address, it keeps the packet local or forwards it to the appropriate segment. If the bridge does not recognize the address, it forwards the packet to all segments except the one through which the packet arrived.
The bridge first either recognizes the packet's MAC-sublayer address, or it does not, and then it forwards the packet appropriately. Figure 7.16 shows a bridge and a router and how they relate to the OSI reference model.
Figure 7.16 Bridges work at the data-link layer MAC sublayer, and routers work at the network layer
Broadcasting Forwarding the packet is the key to understanding bridges and distinguishing them from routers. With bridges, forwarded broadcast data goes out to every computer from all ports of the bridge except the one through which the packet arrived. That is, each computer on all networks (except the local network from which the broadcast originated) receives a broadcast packet. In small networks this might not have much of an impact, but a large network can generate enough broadcast traffic to slow down a network even though it is filtering for network addresses.
The router, which works at the network layer, takes more information into account than the bridge does, determining not only what to forward but where to forward it. The router recognizes not only an address, as the bridge does, but a type of protocol as well. Additionally, the router can identify the addresses of other routers and determine which packets to forward to which routers.
Multiple Paths A bridge can recognize only one path between networks. A router can search among multiple active paths and determine which is the best path at that particular moment.
As illustrated in Figure 7.17, if router A has a transmission that needs to be sent to router D, it can send the message to router C or to router B, and the message will be forwarded to router D. Routers have the ability to evaluate both paths and determine which would be the best route for that transmission.
Figure 7.17 Routers recognize and use multiple paths between destinations
Conclusion Four key pieces of information can help you distinguish between a bridge and a router and determine which would be appropriate in a given situation:
The bridge recognizes only local MAC-sublayer addresses (the addresses of NICs in its own segment). Routers recognize network addresses.
The bridge broadcasts (forwards) everything it does not recognize and forwards all addresses it knows, but only from the appropriate port.
The router works only with routable protocols.
The router filters addresses. It forwards particular protocols to particular addresses (other routers)
ennum... Gateways dus...
Gateways
Gateways enable communication between different architectures and environments. They repackage and convert data going from one environment to another so that each environment can understand the other environment's data. A gateway repackages information to match the requirements of the destination system. Gateways can change the format of a message so that it conforms to the application program at the receiving end of the transfer. For example, electronic-mail gateways, such as the X.400 gateway, receive messages in one format, translate it, and forward it in X.400 format used by the receiver, and vice versa.
A gateway links two systems that do not use the same:
Communication protocols.
Data-formatting structures.
Languages.
Architecture.
Gateways interconnect heterogeneous networks; for example, they can connect Microsoft Windows NT Server to IBM's Systems Network Architecture (SNA). Gateways change the format of the data to make it conform to the application program at the receiving end.
How Gateways Work
Gateways are task-specific, which means that they are dedicated to a particular type of transfer. They are often referred to by their task name (Windows NT Server to SNA gateway).
As shown in Figure 7.18, a gateway takes the data from one environment, strips off its old protocol stack, and repackages it in the protocol stack from the destination network.
To process the data, the gateway:
Disassembles incoming data through the network's complete protocol stack.
Encapsulates the outgoing data in the complete protocol stack of the other network to allow transmission.
Figure 7.18 Gateways strip off an old protocol stack and add a new protocol stack
Some gateways use all seven layers of the OSI reference model, but gateways typically perform protocol conversion at the application layer. However, the level of functionality varies widely between types of gateways.
Mainframe Gateways
One common use for gateways is to act as translators between personal computers and minicomputer or mainframe environments. A host gateway connects LAN computers with mainframe and minicomputer systems that do not recognize intelligent computers attached to LANs.
In a LAN environment, as shown in Figure 7.19, one computer is usually designated as the gateway computer. Special application programs in the desktop computers access the mainframe by communicating with the mainframe environment through the gateway computer. Users can access resources on the mainframe just as if these resources were on their own desktop computers.
Figure 7.19 Mainframe gateways connect personal computers to mainframes
Gateway Considerations
Typically, gateways are dedicated servers on a network. They can use a significant percentage of a server's available bandwidth because they are carrying out resource-intensive tasks such as protocol conversion. If a gateway server is used for multiple tasks, adequate RAM and CPU bandwidth needs to be allocated or performance of the server functions will be degraded.
Gateways are considered as choices for implementation because they do not put a heavy load on internetwork communication circuits, and they perform specific tasks efficiently.
Kleine samenvatting op de diverse apparaten in netwerkwereld:
The following points summarize the main elements of this lesson:
Modems make it possible to communicate over telephone lines.
There are two types of modems: synchronous and asynchronous.
It is important to choose the right cable when connecting hubs; crossover cables will not work in place of standard patch cables.
Asymmetric digital subscriber line (ADSL) is a technology for increasing the speed of transmission on telephone lines.
Repeaters are used to connect two segments of similar or dissimilar media and to regenerate a signal to increase the distance transmitted.
Repeaters should not be used where network traffic is heavy, segments are using different access methods, or filtering is needed.
Bridges have all the features of repeaters.
Bridges are used to connect two segments to expand the length or number of nodes on the network, to reduce traffic by segmenting the network, or to connect dissimilar networks.
Routers are used to connect two networks, limit unnecessary traffic, and to separate administrative networks.
Brouters combine the features of bridges and routers; a brouter can act as a router for one protocol and as a bridge for all the others.
Gateways perform protocol and data conversion.
Gateways are limited in several ways: they are task-specific, expensive, and can be slow.
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