The most common Layer 3, or network layer, protocols in use on LANs are Internet Protocol (IP), IPX, and AppleTalk. IP, sometimes called TCP/IP, is an open standard protocol that is defined and developed by an organization called the Internet Engineering Task Force (IETF). The standards that define IP are distributed in the form of Request for Comment (RFC) documents that are freely available from many sites on the Internet. IP, IPX, and AppleTalk are all routable protocols and thus effective for large-scale networking.
Nonroutable protocols such as NetBEUI, SNA, and the older LAT protocol pose serious scalability problems to a LAN because they require that all segments sharing resources be bridged together. Breaking up broadcast domains (network regions interconnected by repeaters or bridges) by using routable protocols leads to much more efficient networks.
There are other routable protocols in use on LANs, such as the Banyan Vines VIP protocol. Banyan Worldwide officially changed its name to ePresence Solutions in 2000 and dropped support for all Banyan products in 2001 to become a service-centered, rather than a product-centered, company. Thus, this protocol is effectively obsolete and should be avoided in any LAN architecture.
Sometimes you'll encounter a handful of other routable protocols, such as DECNET and OSI. DECNET was used primarily by equipment made by Digital Equipment Corporation. When that company broke up, most organizations that had used DECNET began to migrate away from it. It can still be found in some networks, however. OSI, on the other hand, is a general purpose routable protocol that was once championed as the next great thing in networking. But it never quite managed to secure a foothold in the networking marketplace.
Over the last several years, IP has been replacing these other protocols as the favorite option of many network designers. This growth of IP has been fueled by a number of factors, particularly the public Internet, which uses IP exclusively. Accompanying this growth has been a steady development of new features for IP. Features such as DHCP, VRRP/HSRP, multicast, and Quality of Service capabilities have effectively eliminated the technological advantages of some of these other protocols. Today, the only reason to consider other protocols is for compatibility with legacy applications.
I discuss IPX design issues in Chapter 7. IPX has some particularly interesting properties that affect how it is used in a LAN, and I still frequently encounter it in large LANs. AppleTalk, on the other hand, is a topic that would require an entire book of its own to do it justice. Its breadth puts it beyond the scope of this book.
IP is the protocol that the Internet uses, appropriately enough. Most comments I make in this section are specific to IPv4. A newer version, called IPv6 (or sometimes IPNG for "next generation"), is not yet in wide use. It seems likely that one day IPv6 will supplant the current IPv4 as the dominant version. I discuss IPv6 in more detail in Chapter 10.