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IP Variable Length Subnet Masking (VLSM)
(Page 3 of 3)
An Example: Multiple-Level Subnetting Using VLSM
VLSM subnetting is done the same
way as regular subnetting; it is just more complex because of the extra
levels of subnetting hierarchy. You do an initial subnetting of the
network into large subnets, and then further break down one or more
of the subnets as required. You add bits to the subnet mask for each
of the "sub-subnets" and "sub-sub-subnets" to reflect their smaller
size. In VLSM, the slash
notation of classless addressing is commonly
used instead of binary subnet masksVLSM is very much like CIDR
in how it worksso that's what I will use.
Note: Before proceeding to the VLSM example that follows, a suggestion: if you aren't feeling comfortable with how basic subnetting works, you probably want to read through the section on practical subnetting first. Trust me. J
Let's take our example above again
and see how we can make everything fit using VLSM. We start with our
Class C network, 220.127.116.11/24. We then do three subnettings as follows
(see Figure 72
for an illustration of the process):
- We first do an initial subnetting by
using one bit for the subnet ID, leaving us 7 bits for the host ID.
This gives us two subnets: 18.104.22.168/25 and 22.214.171.124/25. Each
of these can have a maximum of 126 hosts. We set aside the first of
these for subnet S6 and its 100 hosts.
- We take the second subnet, 126.96.36.199/25,
and subnet it further into two sub-subnets. We do this by taking one
bit from the 7 bits left in the host ID. This gives us the sub-subnets
188.8.131.52/26 and 184.108.40.206/26, each of which can have 62 hosts.
We set aside the first of these for subnet S5 and its 50 hosts.
- We take the second sub-subnet, 220.127.116.11/26,
and subnet it further into four sub-sub-subnets. We take 2 bits from
the 6 that are left in the host ID. This gives us four sub-sub-subnets
that each can have a maximum of 14 hosts. These are used for S1, S2,
S3 and S4.
Figure 72: Variable Length Subnet Masking (VLSM) Example
This diagram illustrates the example described in the text, of a Class C (/24) network divided using three hierarchical levels. It is first divided into two subnets; one subnet is divided into two sub-subnets; and one sub-subnet is divided into four sub-sub-subnets. The resulting six subnets are shown with thick black borders, and have a maximum capacity of 126, 62, 14, 14, 14 and 14 hosts.
Okay, I did get to pick the
numbers in this example so that they work out just perfectly, but you
get the picture. VLSM greatly improves both the flexibility and the
efficiency of subnetting. In order to use it, routers that support VLSM-capable
routing protocols must be employed. VLSM also requires more care in
how routing tables are constructed to ensure that there is no ambiguity
in how to interpret an address in the network.
As I said before, VLSM is similar
in concept to the way classless
addressing and routing (CIDR) is performed.
The difference between VLSM and CIDR is primarily one of focus. VLSM
deals with subnets of a single network in a private organization. CIDR
takes the concept we just saw in VLSM to the Internet as a whole, by
changing how organizational networks are allocated by replacing the
single-level classful hierarchy with a multiple-layer hierarchy.
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Version 3.0 - Version Date: September 20, 2005
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