2.1 Classfull Addressing
2.1.2 Advertising Network Addresses
Each of the hosts in the Internet needed to be uniquely identifiable. In the Internet's two-level hierarchy, this required an address with two parts:
  • Network address
  • Host address

Together, these two types of addresses could uniquely identify any and all machines connected via the Internet. It is possible that the needs of a small, networked community could be satisfied with just host addresses, as is the case with LANs. Network addresses, however, are necessary for end systems on different networks to communicate with each other. It is the unique combination of both the host and network addresses that make it possible to access any given host in an internetwork.

The Internet uses this two-level address hierarchy. Rather than calculating and tracking routes to each known host, however, the Internet advertises just the network addresses. End systems that need to access hosts in other networks address their datagrams with the full address, including both network and host numbers, but the routers in the internetwork could assume that the destination network would know how to deliver datagrams to all the end systems within its domain. Therefore, the Internet's routers would only have to track routes to every known network.

A router in the Internet's backbone would quickly become overwhelmed if it tracked paths through the Internet to each end system or host. Instead, the architects of the Internet and IP implemented a two-tiered physical architecture. This was accompanied by a two-tiered network address, consisting of the network's address and a host address. The most practical implication of such a scheme was that routers in the Internet's backbone could greatly reduce their workload by just tracking routes to network addresses.

To understand how network routes are advertised, consider the diagram in Figure . This diagram shows three networks: A, B, and C. Each has a small number of hosts that are numbered numerically. All the routers have host address 1 in their network number. Therefore, the router that interconnects Network A to the Internet has the address A.1.

The routers that comprise the Internet's backbone could calculate routes through the Internet for each of the hosts in Figure . Figure presents their routing tables (in a highly simplified and homogenized form).

As can be seen in Figure , the destination gateway identified by Internet routers wouldn't vary by host address. Therefore, tracking routes to individual hosts (known as host-based routing) would only create unnecessary work for the routers in the internetwork. The destination gateway would, however, vary by network address. Therefore, the Internet's routers can reduce their workload (and increase their efficiency) by not remembering routes to every host. They can advertise routes to network numbers without compromising their ability to deliver datagrams. Figure demonstrates how network route advertising can reduce the size of routing tables.

Note: It does not make sense to program individual host routes because it doesn't scale. And that building route tables based on definitions of networks (which include all hosts on that network) is much more scaleable.

The greatly reduced size of the network-based routing tables does not compromise the capability of the Internet routers to forward datagrams to their destinations. Advertising network routes does, however, have numerous other implications. It can improve a router's performance, for example. The more entries in a router's table means it takes less time to determine where to forward a datagram.

However, the larger a network's routing tables become, the greater the demands that are placed on the network routers' physical resources. These include random access memory (RAM) and central processing unit (CPU) cycles. Providing increasing amounts of both resources can become expensive.

Closely related to performance, but much more significant, is scalability. Network advertisement enables internetworks to be highly scalable. Proof of this is evident in the mammoth proportions of today's Internet. Without the capability to advertise network routes, the Internet's growth would have been severely constrained.

The Internet was aided in its scalability by its sophisticated address architecture. Its architects foresaw the potential for its growth and developed an addressing architecture that was both flexible and extensible. This architecture was implemented in IP.