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Public Addresses

Differing from the private addresses, public addresses are assigned and unique throughout the Internet. Unfortunately, under IP v4 and the methods used to assign addresses, there is a shortage of address space, especially in the larger network allocations—Classes A and B.

There should be little surprise that the advantages of RFC 1918 addresses are the disadvantages of public addresses, given the binary nature of selecting public or private address space. The corollary is also true.

The most significant negative of private addresses is that they are private. Anyone in any company can select any of them to use as they see fit. Some would argue that the benefits of returning IP addresses to the public pool to address the negatives are worth the complexities, including address translation and proxying Internet connections. However, consider the impact when two corporations not using RFC 1918 addresses merge in the context of the following:

  NAT and proxies are not needed.
  Protocols that do not support NAT, including NetBIOS, can traverse the network without difficulty.
  Designers are assured that their addresses are unique. This may become an issue following the merger of two companies that selected addresses under RFC 1918.
  Troubleshooting is simplified because Layer 3 addresses do not change during a host-to-host connection.

When corporations merge, they ultimately will merge data centers and resources to reduce operating costs. This will typically require readdressing for at least one of the two merged organizations if there is overlap. In addition, it is atypical for two design teams to allocate addresses exactly the same way. For example, architect one may place routers at the top of the address range, while architect two may prefer the bottom. Both ways are valid, but upon integration this minor difference may cause problems for support staffs and administrators.

The Function of the Router

The router is designed to isolate the broadcast domain and divide networks on logical boundaries—a function of the OSI model’s Layer 3. This differs from switches and bridges, which operate at Layer 2, and repeaters and hubs, which operate at Layer 1.

Today’s routers provide many additional features for the network architect, including security, encryption, and service quality. However, the role of the router remains unchanged—to forward packets based on logical addresses. In network design, this is considered routing.


The router provides two different functions in the network beyond the simple isolation of the broadcast domain. First, the router is responsible for determining paths for packets to traverse. This function is addressed by the routing protocol in use and is considered overhead. The dynamic updates between routers are part of this function.

The second function of the router is packet switching. This is the act of forwarding a packet based upon the path-determination process. Switching encompasses the following:

  Entry of the packet into the router.
  Obtaining the address information that will be needed for forwarding the packet. (In ATM, or Asynchronous Transfer Mode, it is the cell’s VPI/VCI, or virtual path identifier/virtual channel identifier.)
  Determining the destination based on the address information.
  Modifying the header and check sum information as necessary.
  Transmitting the packet/frame/cell toward its destination.

While the router may also handle additional services, this list describes the functional steps required by the forwarding process. In addition to the forwarding of packets based on the Layer 3 logical address, the router is also required to determine the routes to those destinations—a process that relies on the administrative distance function described in the next section. However, routing, or more accurately, administration of the router, requires designers to consider many factors. Addressing, routing protocols, access lists, encryption, route maps (manipulation of the routing tables), and router security will only demand more attention in future years. Paths will also incorporate mobile IP and VPN (Virtual Private Network) technologies as the concept of an 80/20 rule migrates through 20/80 and toward 2/98. This means that virtually no traffic will remain local to the subnet, and as a result, the demands on administrators to work with other service providers will also increase.

If the router does not have a local interface in the major network and it receives a routing update with a classful protocol, the router will presume the natural mask. The natural mask for Class A is; for Class B it is; and for Class C it is Readers should make sure that they understand how to identify an address’ class and what the natural mask would be before continuing. This subject is covered in greater detail in the CCNA and ICRC preparation materials.

Administrative Distance

A router performs its function by determining the best method to reach a destination—a function that relies on the routing table and metrics. Metrics will be reviewed in greater detail in Chapter 4, but for now the metric of hops used in the IP RIP protocol will be our basis. You may recall that IP RIP adds a hop to each route when it passes through a router. Therefore, a source router can compare two or more routes to the same destination and typically presumes that the lowest hop count determined by the routing protocol will correspond with the best path through the network. Chapter 4 will discuss the limitations of the hop-based methodology; however, this system works reasonably well for links of similar bandwidth.

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