User`s guide

184 Appendix E Understanding IP Addresses and Subnet Masking
The subnet structure of a network cannot be seen outside an organization's private network. The
route from the Internet to any subnet of a given IP address is the same, no matter which subnet
the destination host is on. This is because all subnets of a given network number use the same
network prefix, but different subnet numbers. Devices like the XStack Storage, switches, and
routers within an organization need to differentiate between the individual subnets. However, as
far as the Internet routers are concerned, all subnets in a company are collected into a single
routing table entry. This allows a company to introduce arbitrary complexity into its private
network, without affecting the size of the Internet’s routing tables.
E.5 Variable Length Subnet Masks
A key limitation of supporting only a single subnet mask across a given network prefix (as shown
in the previous examples) is that once the mask is selected, it locks the organization into a fixed-
number of fixed-sized subnets.
To further illustrate this point, assume that a network administrator decided to configure the
130.5.0.0/16 network with a /22 extended-network prefix. This arrangement permits 64 subnets,
each of which supports a maximum of 1,022 hosts (2
10
-2). This is fine for deploying a number of
large subnets, but not if an organization wants a small subnet containing only 20 or 30 hosts.
Since a subnetted network can have only a single mask, the 20 or 30 hosts still have to be
assigned to a subnet with a 22-bit prefix. This wastes approximately 1,000 IP host addresses for
each small subnet deployed and makes for an inefficient use of an organization's address space.
Variable length subnet masks (VLSMs) allow a subnetted network to be assigned more than one
subnet mask. Assume that in the previous example, the organization is also allowed to configure
the 130.5.0.0/16 network with a /26 extended-network prefix. A /16 network address with a /26
extended-network prefix permits 1024 subnets (2
10
), each of which supports a maximum of 62
hosts (2 6 -2). The /26 prefix would be ideal for small subnets with less than 60 hosts, while the
/22 prefix is well suited for larger subnets containing up to 1000 hosts.
The following example shows how VLSMs can be used. This example assumes that an organization
has been assigned the network number 140.25.0.0/16 and plans to deploy VLSM.
1. The first step in the subnetting process divides the base network address into 16 equal-size
address blocks. Since 16 = 2 4, four bits are required to uniquely identify each of the 16
subnets. This means four more bits, or a /20, must be used in the extended-network prefix to
define the 16 subnets of 140.25.0.0/16. Each subnet represents a contiguous block of 2 12 (or
4,096) network addresses.
The 16 subnets of the 140.25.0.0/16 address block are shown below. The subnets are
numbered 0 through 15. The italicized portion of each address identifies the extended-
network prefix, while the bold digits identify the 4-bits representing the subnet-number
field: