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.
