Almost all networks-even small ones-need to connect to the outside world. Internet connectivity is the most common need. If you are a branch office of a larger company, then you may need the ability to connect to the parent organization's network.

Connecting to external networks is one of the most complex parts of designing and implementing a network. Most small networks avoid the complexities of dealing with routers for their local network, but some type of routing device will be involved to connect with the outside world. Fortunately, there are some relatively simple devices available that have been specifically designed for small networks.

Most small organizations that need to connect to external networks will be able to get assistance from the group that manages the target network. In order to connect to the Internet, you will need to work with an ISP (Internet service provider) that will guide you through the process. If you are connecting to your company's home office network, there also likely will be data communications staff on board to lend a hand.

One of the most challenging aspects of implementing an external connection is determining how fast a link to install and which of the available technologies to use. The challenge lies in purchasing enough bandwidth to provide adequate performance at the lowest reasonable cost. Connectivity costs can vary from $30 per month to $20,000 a month, so you want to choose the best value for your organization's level of use.

At the low end of the market, you can rely on single computer dial-up access. With this approach, you simply install a modem in each computer and use regular analog phone lines to connect to an ISP. The main disadvantage of this approach lies in the contention for the phone lines between voice and data, and the cumulative cost of all the additional phone lines that would end up being dedicated to modem use. This model of access, though common for the one- or two-person office, is generally not sustainable for most small businesses that have at least a dozen employees. A major limitation of this approach is that it does not support the ability to provide services on the Internet. If you want to have a Web server or other resource that can be accessed on the Internet, then you need a full-time dedicated connection.

The more functional model of connecting to the Internet involves establishing a link between your local network and the Internet, rather than providing Internet access to individual computers. To enable this model of connectivity you would use a device called an router. Large networks have sophisticated routers that manage multiple internal and external connections. Such a router can easily cost $20,000 and requires an experienced network engineer to program. Small networks typically use much simpler, scaled-down devices called access routers. An access router would have an Ethernet port that connects to your local network and another port that connects to the link to your ISP and is equipped with simplified routing software that can be easily configured.

Routers are devices that help networks connect efficiently. We noted earlier that networks pass information in packets. A router examines each packet on the network and makes decisions about the quickest way to get it to its destination. Routers are protocol specific-different routing rules can be established for IP, IPX and the like. For Internet access, IP routing is all that is necessary. Even if you use IPX within your local network, you can use an IP-only access router, as long as your workstations support it. The router examines each IP packet, and if the destination address does not fall within the local network, it is forwarded to the ISP.

The world of Internet connectivity changes quickly. Until recently standard data communications links such as ISDN and frame relay have been dominant, but newer methods such as cable modems are emerging. To make a choice for your organization, you will need to explore all the options available in your area.

One very popular approach for connecting a small network to the Internet or a home office network involves the use of ISDN telephone circuits. ISDN supports both voice and data, and it relies on digital communications (standard phone service is analog). A Basic Rate ISDN line consists of two data-bearing (D) channels of 64 Kbps each and a delta (D) channel of 16 Kbps that controls the circuit. Most ISDN access routers can combine the two B channels into a single 128 Kbps datastream. ISDN also allows you to use one channel for voice calls while the other carries data. ISDN can be a relatively inexpensive way to achieve dedicated full-time Internet access, but the pricing for ISDN varies among geographical areas. Most-but not all-areas price ISDN at a flat monthly rate and do not assess per-minute access charges. If you have to pay for each minute of access, then ISDN probably will not be cost effective and you will need to consider alternatives. Again, most, but not all, ISPs support ISDN.

Frame relay is also used by many small business networks. With frame relay, you purchase a telephone circuit that connects to the telephone company's network, which in turn connects to your service provider. Options for frame relay start at about 64 Kbps, and links of 256 Kbps are quite common for small and midsize networks. Organizations with more strenuous connectivity needs will want to consider T1 or Fractional T1 links. Extremely large networks may have OC3 or OC12 links to the Internet, but these capacities are far beyond the needs of the small business network.

To connect your network to the Internet, the only relevant protocol is TCP/IP. No other protocol is routed on the Internet. Through schemes such as IP tunneling, you can connect IPX-based NetWare LANs through the Internet, but this is a relatively rare practice.

When connecting to a home office network, you have options for routing other protocols. If you are part of an organization that uses NetWare, then it is likely that you will want to implement the ability to route IPX when you establish a link to that network. Most of the midrange routers offer the ability to route IPX, but it may be an optional feature. Most low-end access routers do not route IPX. You will need to work with your network administrator to configure your router properly to manage traffic between your networks. How you assign IP network addresses and the IPX network numbers is extremely important if you plan to connect your network to external networks. In most cases the larger network will designate the various network addresses that you must use. Similar considerations hold if you're using the AppleTalk protocol with MacOS computers.