$80$6)0ROAND$80($-)3ERIESs2EFERENCE)NFORMATION
132
132
Subnet masks and octets
The subnet mask (figure
120
) is used to determine whether the local and remote devices
are on the same subnet or different subnets. The subnet mask consists of four numeric
OCTETSSEPARATEDBYDOTS%ACHOCTETCANBENUMBEREDFROMTHROUGH,EADING
ZEROSUPTOTHREEDIGITSTOTALPEROCTETAREOPTIONAL%ACHOCTETTYPICALLYCONTAINSEITHER
255 or 0. The octets determine whether or not the same octets of two IP addresses will be
compared when determining if two devices are on the same subnet.
255.255.0.0
Typical Subnet Mask:
Octets
255 indicates that this octet will be
compared between two IP addresses.
0 indicates that this octet will
not
be
compared between two IP addresses.
Figure 120.
Subnet Mask and Octets
Determining whether devices are on the same subnet
To determine the subnet, the local device IP address is compared to the remote device
IP address (see figure
121
, below). The octets of each address are compared or not,
depending on the value in the related subnet mask octet.
s
If a subnet mask octet contains the value 255, the related octets of the local device
address and the remote device IP address are unmasked.
Unmasked octets are compared
(indicated by
?
in figure
121
).
s
If the subnet mask octet contains the value 0, the related octets of the local device
and remote device IP addresses are masked.
Masked octets are not compared
(indicated by
n
in figure
121
).
If the unmasked octets of the two IP addresses
match
(indicated by
=
in example 1 of the
figure below), the two addresses
are
on the same subnet
.
If the two unmasked fields
do not match
(indicated by an unequal sign
in the
figure below, examples 2 and 3), the addresses
are not on the same subnet
.
192.168.254.254
255.255.0.0 (
?
.
?
.
X
.
X)
192.168.2.25
= . = .
X
.
X
—
Match
(Same subnet)
(Different subnet)
(Different subnet)
Local IP Address:
Subnet Mask:
Remote IP Address:
Match?:
Example 1
Example 2
192.168.254.254
255.255.0.0 (
?
.
?
.
X
.
X)
190.190.2.25
≠
.
≠
.
X
.
X
—
No match
Example 3
192.168.254.254
255.255.0.0 (
?
.
?
.
X
.
X)
192.190.2.25
=
.
≠
.
X
.
X
—
No match
Figure 121.
Comparing the IP Addresses
