Implementing Variable-Length Subnet Masks, Best CCSP Training Institute in Delhi Gurgaon

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Variable-length subnet masks (VLSM) were developed to enable multiple levels of subnetworked
IP addresses within a single network. This strategy can be used only when it is supported by the
routing protocol in use, such as Routing Information Protocol version 2 (RIPv2), OSPF, and
EIGRP. VLSM is a key technology on large, routed networks. Understanding its capabilities is
important when planning large networks.
Reviewing Subnets
Prior to working with VLSM, it is important to have a firm grasp on IP subnetting. When you are
creating subnets, you must determine the optimal number of subnets and hosts.
Computing Usable Subnetworks and Hosts
Remember that an IP address has 32 bits and comprises two parts: a network ID and a host ID.
The length of the network ID and host ID depends on the class of the IP address. The number of
hosts available also depends on the class of the IP address.
The default number of bits in the network ID is referred to as the classful prefix length. Therefore,
a Class C address has a classful prefix length of /24, a Class B address has a classful prefix length
of /16, and a Class A address has a classful prefix length of /8. This is illustrated in Figure 3-29.
Figure 3-29 Classful Prefix Length
Class C Network Network Network
. . .
Bits to Borrow
Class B Network Network
. . .
Bits to Borrow
Class A Network
. . .
Bits to Borrow
124 Chapter 3: Medium-Sized Routed Network Construction
The subnet address is created by taking address bits from the host-number portion of Class A,
Class B, and Class C addresses. Usually a network administrator assigns the subnet address
locally. Like IP addresses, each subnet address must be unique.
Each time one bit is borrowed from a host field, one less bit remains in the host field that can be
used for host numbers, and the number of host addresses that can be assigned per subnet decreases
by a power of 2.
When you borrow bits from the host field, note the number of additional subnets that are being
created each time one more bit is borrowed. Borrowing two bits creates four possible subnets
(22 = 4). Each time another bit is borrowed from the host field, the number of possible subnets
increases by a power of 2, and the number of individual host addresses decreases by a power of 2.
The following are examples of how many subnets are available, based on the number of host bits
that you borrow:
■ Using 3 bits for the subnet field results in 8 possible subnets (23 = 8).
■ Using 4 bits for the subnet field results in 16 possible subnets (24 = 16).
■ Using 5 bits for the subnet field results in 32 possible subnets (25 = 32).
■ Using 6 bits for the subnet field results in 64 possible subnets (26 = 64).
In general, you can use the following formula to calculate the number of usable subnets, given the
number of subnet bits used:
Number of subnets = 2s (in which s is the number of subnet bits)
For example, you can subnet a network with a private network address of 172.16.0.0/16 so that it
provides 100 subnets and maximizes the number of host addresses for each subnet. The following
list highlights the steps required to meet these needs:
■ How many bits will need to be borrowed?
— 2s = 27 = 128 subnets (s = 7 bits)
■ What is the new subnet mask?
— Borrowing 7 host bits = 255.255.254.0 or /23
■ What are the first four subnets?
— 172.16.0.0, 172.16.2.0, 172.16.4.0, and 172.16.6.0
Implementing Variable-Length Subnet Masks 125
■ What are the ranges of host addresses for the four subnets?
— 172.16.0.1–172.16.1.254
— 172.16.2.1–172.16.3.254
— 172.16.4.1–172.16.5.254
— 172.16.6.1–172.16.7.254