From Bridges to Routers continue...

Using a Router to Segment the Network

Like bridges, routers can be used to isolate traffic between network segments. Unlike bridges, routers further reduce network bandwidth use because they do not pass broadcast messages from one segment to another unless programmed to do so. A router also does not have to take time to learn which nodes are connected to each segment. The information it needs is configured in advance—the administrator assigns protocols and addresses to each port. Routing protocols also use various methods to update each other about network topology as it changes.

One very important reason why routers are used to help organize a network into segments is that routers enable you to connect many network segments. Whereas bridges are limited to a few thousand nodes, depending on the topology used, routers can enable the LAN to be connected to an infinitely larger WAN, such as the Internet.

The internal processing that routers must perform make them slower than bridges (although that might not be the case with most high-end routers being manufactured today), which need to examine only a small amount of data in the packet header. Although this performance difference will not be noticed on network segments with only moderate traffic use, you might find that you need to place routers at only strategic locations throughout the network, retaining switches for connecting other computers or network segments. Remember that you can connect individual computers to a switch port or use the port to connect to other switches.

The method you use will depend on the usage patterns that can be monitored for each segment and the cost of the links used to connect different segments. Another thing to consider is that many of the high-end routers available today operate at what is called "wire speed." That means they can route packets at virtually the same speed as the network medium, with just the very slightest delay for processing time.

Connecting to a Larger WAN or the Internet

When connecting the LAN to a WAN, a router is required. When connecting to the Internet, for example, you cannot use a bridge or a repeater. The Internet is composed of a hierarchical IP address space and a router is needed to participate in this hierarchy. Or you might plan to use a dedicated line of some sort to connect to a larger corporate network. In that case, placing a router between your LAN and the WAN hardware, such as an ATM switch connection, will help reduce the traffic that crosses the expensive dedicated connection by keeping local traffic confined to the local network segments.

Note

There are two situations in which a router is not needed to make an Internet connection. The first is if you have a modem- based dial-up connection. Although it's possible to set up routing tables in operating systems such as Windows and Unix/Linux, this isn't really a practical method for connecting a small office IAN to the Internet due to the very limited speed.

The other situation is when you use a broadband connection, such as a cable or DSL modem. In this case, you can connect the high-bandwidth modem to a single computer and then set up routing tables so that other computers can send and receive traffic through the computer, which operates as a router. However, there's a better idea in a Small Office/Home Office (SOHO) or Remote Office Branch Office (ROBO) environment or a home environment where everyone from the parents to the kids have their own computers: Purchase an inexpensive 4-6 port router (for less than $100 in mast cases) that you can connect to the cable/DSL modem. These types of routers require very little knowledge about computers and can usually be set up in less than a half-hour.

Although you'll certainly have to configure the ports that connect the local LAN and the WAN interface, you might have to reconfigure addressing information on clients. For example, if you're already using a valid TCP/IP network address, possibly a subnet of the corporate network address space, you'll need to configure only routers.

If your business has just been acquired by a larger concern, however, you might find that your LAN has been assigned a new subnet by the larger corporation. In such a case, you'll probably have to plan on downtime for end users in order to make changes to important servers, such as Domain Name System (DNS) servers. DHCP servers (which workstations can find themselves) are used to translate between user-friendly names and IP addresses. Although DHCP can dynamically assign configuration information to workstations, important servers, such as DNS servers or gateways to other networks (usually routers), must have a static (unchanging) address. This is because part of the configuration information that DHCP supplies to clients is those addresses! If the address of a DNS server changed with every reboot of the server, you would have to reconfigure the information on each workstation client—a tedious effort even in a small network!

By using DHCP, you can overcome client configuration headaches such as this. Just reconfigure the DHCP server with the address range for the new subnet, add in the DNS servers and default gateway, and reboot your client computers. This is a simple explanation of the information supplied by DHCP servers. Indeed, you can use DHCP to provide configuration information for many other network parameters.

Growing Beyond a Small LAN

The basic devices used to interconnect network segments: repeaters, bridges, switches, and routers. Each of these devices builds on the one previous to it so that together they span a continuum of functionality that you can use to solve problems with a LAN or WAN. Standard bridges were developed to enable you to extend the reach of a LAN and to limit traffic to local segments, therefore building on the function of repeaters. Switches took this concept further by enabling each workstation or server to have its own physical LAN segment, thus limiting the broadcast domain (just the workstation and the switch). Routers enable you to extend the reach of a LAN by connecting it to a wide area network.

Note

A switch that operates in full-duplex mode eliminates the collision domain between the switch port and the device attached to it. Instead of using the same set of wires for transmitting and receiving data (half-duplex), full-duplex switches use separate wires for transmitting and receiving, so both ends of the path can be sending information at the same time. Thus, a switch port and network card operating in full-duplex mode can essentially double the network bandwidth.

To quickly summarize:

1. Repeaters are simple devices that connect network segments (usually two segments). They repeat all traffic by regenerating and attenuating a signal, allowing the standard distance limitation to be extended. They do not, however, do anything to help segment network traffic patterns. Repeaters are used to expand a LAN when it grows beyond the limitations imposed by a single network segment. Multiport repeaters function in the same way, but resemble a hub in that more than one segment can be connected to a multiport repeater. However, multiport repeaters are typically used in much older environments that use coaxial cables for the network media and use BNC connectors. Most hubs have RJ-45 jacks and receive twisted-pair wiring with RJ-45 modular connectors. If your network still uses repeaters of this type, you're long overdue for an upgrade.

2. Bridges are similar to repeaters except that they apply a little intelligence to the packet- forwarding process: Bridges learn MAC addresses of devices on each segment when they make an initial transmission. From then on, a bridge will not pass traffic to another segment if it knows the recipient is on the segment local to the transmission. Bridges are helpful for expanding a LAN and can be used to group collections of computers and servers that commonly interact to lower overall bandwidth consumption.

3. Routers work like bridges in that they're selective about which packets get forwarded on which ports. However, whereas bridges operate at layer 2 of the OSI reference model (the Data Link layer) and look only at the flat namespace provided by the MAC addresses, routers operate at layer 3 (the Network layer) and make decisions based on the addressing scheme provided by a higher-level networking protocol. Bridges are typically used to create larger local area networks. Connecting a LAN to other LANs or to a larger WAN can be done using a router.

4. Switches are the current technology for connecting network LAN segments as well as for connecting individual network nodes to the network. Switches operate like bridges in that they keep track of which network node is located on each port by remembering MAC addresses kept in system memory. When retransmitting an incoming packet, the switch will send it out only on a port that will get it to its destination, provided that it has already learned the destination's MAC address. Whereas bridges usually have only two ports, switches are like hubs and contain many ports. Most switches will allow for full-duplex operation, thus effectively doubling the available network bandwidth for a single node connected on a segment. In a sense, a switch operates like a collection of bridges. And don't forget that you can connect one switch to another to further localize network traffic. A LAN today can consist of multiple layers of switches that eventually connect to a router.

From this summary, you can see that it's easy to use repeaters or bridges to grow the small LAN, but when it becomes necessary to expand beyond certain limits or when it becomes necessary to make a connection to a larger LAN, you must incorporate routers or switches. Growth is not the only reason you might want to use a router or switch, however. These devices also can be used in a small LAN. For example, a small LAN that's experiencing network traffic congestion might find relief by replacing the hubs in the LAN with switches to cut down on the overall network traffic. Indeed, if you look at the price of a switch today, the benefits you will achieve in network bandwidth are well worth the price. When users begin to complain about network response time in a network that uses hubs, you should definitely consider replacing hubs with switches.

In addition to connecting LANs to larger networks such as the Internet, routers can be used in a campus LAN to allow network administrators to logically group network segments using the addressing scheme provided by TCP/IP (subnetting), for example.