Specifications

Installation and Operation Manual Appendix C Operating Environment
FCD-E1LC Ver. 1.0 IP Environment C-9
Subnetting
Given the scarcity of IP network addresses, for organizations operating several
relatively small, physically separated, IP networks, e.g., several departmental
networks, it is advantageous to enable several physical networks to share a
common IP network address.
Small in this context means that the number of IP
ports connected to each of these networks is small relative to the host address
space for the corresponding IP address class.
The approach taken to enable the sharing of an IP network address by two or
more networks is called
subnetting , which means use of subnets . The subnetting
is relevant only within the using organization, and therefore can be freely
selected to meet its specific needs.
To enable subnetting, the meaning of the bits in the host portion of the IP
address is further sub-divided into two portions:
• Subnet number. For example, subnet numbers can be used to identify
departmental subnets. The subnet number follows the network identifier.
• Host number – the last bits of the IP address.
This subdivision is illustrated below:
Net Number Subnet Number Host Number
For example, when the subnet includes 16 IP hosts, only the last four bits need
to be reserved for the host number. For an organization which obtained one
global Class C network address, this means that four bits are available to identify
subnets. Therefore, this organization can implement 16 IP subnets, each
comprising up to 16 hosts (except for two subnets that are limited to 15 hosts).
Subnet Masks
Subnet masks are used to indicate the division of the IP address bits between the
net and subnet portion and the host portion.
The mask is a 32-bit word that includes “
ones ” in the positions used for net and
subnet identification, followed by “
zeros ” up to the end of the IP address.
For example, the default subnet mask for any Class C address (i.e., all the eight
bits in the host address space are used for hosts in the same net) is
255.255.255.000.
However, if the same address is used in a subnet comprising up to 16 hosts and
for which the host numbers range is 00 to 15, the subnet mask changes as
follows:
IP Address
(Dotted-Quad)
192 70 55 13
IP Address (Binary) 1011 1111 0100 0110 0011 0111 0000 0111
Subnet Mask
(Binary)
1111 1111 1111 1111 1111 1111 1111 0000
Subnet Mask
(Dotted-Quad)
255 255 255 240