Case Study Addressing the Network

For a complicated example, use a Class B address and create an addressing scheme for Mental Merge.

If the Internet assigns the address, how might you address the network shown in the diagram?

The first task is to determine the number of regions, campuses, buildings, floors, and hosts on each floor. You also need to consider any anticipated growth or change in the network.

For this example, the network is comprised of the following:

• Four regions exist, but the company has plans to expand into other areas. Any expansion will probably not exceed eight states (adequate to cover the country).

Within each region/state, there are no more than three campuses.

Figure 3-6

• Within each campus, there are no more than four buildings. This number might increase, however.

• No building has more than three floors.

With this topology and growth detailed, it is possible to start allocating bits of the network address.

Taking the address and writing out the last 16 bits, you can easily assign them to the different addressing tasks at hand. Figure 3-6 covers assigning IP addressing bits for VLSM.

Assigning IP Addressing Bits for VLSM



Building Campus Region



Prefix mask of /27

Consideration must be given to the subnetting rules (RFC 950, "Internet Standard Subnetting Procedure," and RFC 1878, "Variable-Length Subnet Table For IPv4") that state that there must not be all zeros or all ones in the following:

The NIC portion of the address

The subnet portion of the address

The host portion of the address

The algorithm for calculating the number of networks or hosts available is 2n - 2 (where n is the number of bits).

This rule has become complicated recently regarding the subnet portion of the address. The number of subnets is still calculated by the 2n formula, where n is the number of bits by which the subnet mask was extended. However, it is possible to use the all-zero address for the subnet. This makes the formula 2n - 1.

WARNING Although Cisco provides the utility of subnet zero, this command should be used only with full understanding of the network devices and the knowledge that there is no device that uses the zero broadcast. Even today, some systems, such as Sun Solaris 4.x, have problems using subnet zero even with OSPF.

The command to enable the use of the zero subnet became the default configuration in version 12.0 of the Cisco IOS.

Although the 2n - 2 rule is still true for the NIC portion of the address, it is not of concern. This is because the NIC portion of the address is out of your control because it was defined by the Internet.

However, attention must still be given to the host portion of the address. The host portion of the address must conform to the subnet rule as defined; otherwise, it is not possible for the router to distinguish between hosts and broadcast addresses. The host cannot use an IP address of all zeros in the subnet address or all ones in the subnet broadcast address. The all-zeros address is used to show the subnet delimiter.

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