TCP/IP Tutorial and Technical Overview - IBM Redbooks

Therefore, it is normal to use a contiguous block of bits at the beginning of the
local address part for the subnet number. This makes the addresses more
readable. (This is particularly true when the subnet occupies 8 or 16 bits.) With
this approach, either of the previous subnet masks are “acceptable” masks.
Masks such as 255.255.252.252 and 255.255.255.15 are “unacceptable.” In fact,
most TCP/IP implementations do not support non-contiguous subnet masks.
Their use is universally discouraged.
Types of subnetting
There are two types of subnetting: static and variable length. Variable length
subnetting is more flexible than static. Native IP routing and RIP Version 1
support only static subnetting. However, RIP Version 2 supports variable length
subnetting (refer to Chapter 5, “Routing protocols” on page 171).
Static subnetting
Static subnetting implies that all subnets obtained from the same network use the
same subnet mask. Although this is simple to implement and easy to maintain, it
might waste address space in small networks. Consider a network of four hosts
using a subnet mask of 255.255.255.0. This allocation wastes 250 IP addresses.
All hosts and routers are required to support static subnetting.
Variable length subnetting
When variable length subnetting or variable length subnet masks (VLSM) are
used, allocated subnets within the same network can use different subnet
masks. A small subnet with only a few hosts can use a mask that accommodates
this need. A subnet with many hosts requires a different subnet mask. The ability
to assign subnet masks according to the needs of the individual subnets helps
conserve network addresses. Variable length subnetting divides the network so
that each subnet contains sufficient addresses to support the required number of
hosts.
An existing subnet can be split into two parts by adding another bit to the subnet
portion of the subnet mask. Other subnets in the network are unaffected by the
change.
Mixing static and variable length subnetting
Not every IP device includes support for variable length subnetting. Initially, it
appears that the presence of a host that only supports static subnetting prevents
the use of variable length subnetting. This is not the case. Routers
interconnecting the subnets are used to hide the different masks from hosts.
Hosts continue to use basic IP routing. This offloads subnetting complexities to
dedicated routers.
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