The deployment of an addressing plan requires careful thought on the part of the
network administrator. There are four key questions that must be answered before any
design should be undertaken:
1) How many total subnets does the organization need today?
2) How many total subnets will the organization need in the future?
3) How many hosts are there on the organization's largest subnet today?
4) How many hosts will there be on the organization's largest subnet in the future?
The first step in the planning process is to take the maximum number of subnets
required and round up to the nearest power of two. For example, if a organization needs
9 subnets, 23 (or 8) will not provide enough subnet addressing space, so the network
administrator will need to round up to 24 (or 16). When performing this assessment, it
is critical that the network administrator always allow adequate room for future growth.
For example, if 14 subnets are required today, then 16 subnets might not be enough in
two years when the 17th subnet needs to be deployed. In this case, it might be wise to
allow for more growth and select 25 (or 32) as the maximum number of subnets.
The second step is to make sure that there are enough host addresses for the
organization's largest subnet. If the largest subnet needs to support 50 host addresses
today, 25 (or 32) will not provide enough host address space so the network
administrator will need to round up to 26 (or 64).
The final step is to make sure that the organization's address allocation provides enough
bits to deploy the required subnet addressing plan. For example, if the organization has
a single /16, it could easily deploy 4-bits for the subnet-number and 6-bits for the host
number. However, if the organization has several /24s and it needs to deploy 9 subnets,
it may be required to subnet each of its /24s into four subnets (using 2 bits) and then
build the internet by combining the subnets of 3 different /24 network numbers. An
alternative solution, would be to deploy network numbers from the private address
space (RFC 1918) for internal connectivity and use a Network Address Translator
(NAT) to provide external Internet access.
Subnet Example #1
Given
An organization has been assigned the network number 193.1.1.0/24 and it needs to
define six subnets. The largest subnet is required to support 25 hosts.
Defining the Subnet Mask / Extended-Prefix Length
The first step is to determine the number of bits required to define the six subnets. Since
a network address can only be subnetted along binary boundaries, subnets must be
created in blocks of powers of two [ 2 (21), 4 (22), 8 (23), 16 (24), etc. ]. Thus, it is
impossible to define an IP address block such that it contains exactly six subnets. For
this example, the network administrator must define a block of 8 (23) and have two
unused subnets that can be reserved for future growth.
Since 8 = 23, three bits are required to enumerate the eight subnets in the block. In this
example, the organization is subnetting a /24 so it will need three more bits, or a /27, as
the extended-network-prefix. A 27-bit extended-network-prefix can be expressed in
dotted-decimal notation as 255.255.255.224. This is illustrated in Figure 11.
A 27-bit extended-network-prefix leaves 5 bits to define host addresses on each subnet.
This means that each subnetwork with a 27-bit prefix represents a contiguous block of
25 (32) individual IP addresses. However, since the all-0s and all-1s host addresses
cannot be allocated, there are 30 (25 -2) assignable host addresses on each subnet.
No comments:
Post a Comment