221

These bits are borrowed bits by changing the corresponding subnet mask bits to 1s to indi-cate that these bits are now being used as network bits. The last octet of the mask is then represented in binary by 11100000, which is 224. The new mask of 255.255.255.224 is rep-resented with the /27 notation to represent a total of 27 bits for the mask.

In binary, this subnet mask is represented as 11111111.11111111.11111111.11100000. After borrowing 3 of the host bits to use as network bits, this leaves 5 host bits. These 5 bits will allow up to 30 hosts per subnet.

Although you have accomplished the task of dividing the network into an adequate number of subnets, it was done with a significant waste of unused addresses. For example, only two addresses are needed in each subnet for the WAN links. There are 28 unused addresses in each of the three WAN subnets that have been locked into these address blocks. Furthermore, this limits future growth by reducing the total number of subnets available. This inefficient use of addresses is characteristic of fixed-block sizes that is a carryover from practices with classful addressing.

Applying a standard subnetting scheme to this scenario is inefficient. In fact, this example is a good model for showing how subnetting a subnet can be used to maximize address utilization.

Getting More Subnet for Less Hosts

Recall in previous examples that the original subnets were divided to gain additional, smaller subnets to use for the WAN links. Creating smaller subnets, each subnet is able to support two hosts, which leaves the original subnets free to be allotted to other devices and prevents many addresses from being wasted.

To create these smaller subnets for the WAN links in the network in Figure 6-24, begin with 192.168.20.192. You can divide this subnet into many smaller subnets. To provide address blocks for the WANs with two addresses each, you will borrow 3 additional host bits to be used as network bits:

Address 192.168.20.192 is 11000000.10101000.00010100.11000000 in binary.

Mask 255.255.255.252 is 11111111.11111111.11111111.11111100 in binary.

This addressing plan breaks up the 192.168.20.192 /27 subnets into smaller /30 subnets to provide addresses for the WANs. Doing this reduces the number of addresses per subnet to a size appropriate for the WANs. With this addressing, you have subnets 4, 5, and 7 avail-able for future networks, as well as several other subnets available for WANs.