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ControlMatrix User’s Guide - FAQs
11.5 Networking – FAQs
Q1.
How do I know I have correctly configured multiple network devices to
be on the same IP Subnet?
A1.
Both the IP address and the Subnet Mask are 4x 8 bit binary
numbers represented in decimal form. When a computer receives a packet
from a network, part of that packet is the IP header and this is checked
against the computer’s own IP address by testing the bits set by the Subnet
Mask. If the mask bit is 1, the addresses must match. If the mask bit is 0 it
is not tested. Only packets that pass this matching test will be recognised
by the computer.
An example will illustrate the point. For this example assume the PC’s IP
address is 192.168.10.5 and initially the subnet mask is 255.255.255.0 (‘x’
represents a “don’t care” or “doesn’t matter” condition and leading zeros
have been retained in the decimal numbers.)
IP address
192.168.010.005
Subnet mask
255.255.255.000
Matching subnet
= 11000000.10101000.00001010.00000101
= 11111111.11111111.11111111.00000000
= 11000000.10101000.00001010.xxxxxxxx
For this case, any computer with an IP address of 192.168.10.xxx (i.e.
192.168.10.0 through to 192.168.10.255) shares the Subnet. Thus 256
computers may exist on this Subnet. This is due to the fact that the eight
binary bits marked x in the Matching Subnet line are not tested. In decimal,
these 8 binary bits represent 256 – the number of computers that may share
the Subnet.
If the Subnet Mask is now restricted to 255.255.255.248, the above will
change to…
IP address
192.168.010.005
Subnet mask
255.255.255.248
Matching subnet
= 11000000.10101000.00001010.00000101
= 11111111.11111111.11111111.11111000
= 11000000.10101000.00001010.00000xxx
With this restriction, only three bits on the Matching Subnet line are not
tested. The decimal equivalent of three binary bits is eight, so only eight
computers can exist on this subnet and these computers can have IP
addresses in the range 192.168.10.0 to 192.168.10.7 (each of these
correspond to one of the ‘x’ states in the Matching Subnet line). Once the
actual IP address and subnet mask are known, the range of IP addresses
that the computer will recognise can be determined and hence it is possible
to determine if two computers are on the same Subnet.
 Creative Audio, V3.01, July 2003
Page 176