Subnet Masks and Octets
The subnet mask (see figure 74) 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
octets separated by dots. Each octet can be numbered from 000 through 255. Leading
zeroes, up to three digits total per octet, are optional. Each octet typically contains either
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.
255.255.0.0Typical 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 74. Typical Subnet Mask
Determining Whether Devices Are on the Same Subnet
To determine the subnet, the IP address of the local device is compared to the IP address
of the remote device (see figure 75). The octets of each address are compared or not
compared, depending on the value in the related subnet mask octet.
If a subnet mask octet contains the value 255, the related octets of the IP addresses of
the local device and the remote device are unmasked.
Unmasked octets are compared (indicated by ? in figure 75).
If the subnet mask octet contains the value 0, the related octets of the IP addresses of
the local device and remote device are masked.
Masked octets are not compared (indicated by X in figure 75).
If the unmasked octets of the two IP addresses match (indicated by = in figure 75, example 1),
the two addresses are on the same subnet.
If the two unmasked fields do not match (indicated by ≠ in figure 75, example 2 and
example 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 75. Comparing the IP Addresses of the Local and Remote Devices
MPX 866 A Media Presentation Matrix Switcher • Ethernet Connection 119
