QGE 100 • Reference Information 5252
IP addresses and octets
Valid IP addresses consist of four 1-, 2-, or 3-digit numeric sub-fields, called “octets,”
which are separated by dots (periods) (figure 35, below). Each octet can be numbered
from 000 through 255. Leading zeros, up to 3 digits total per octet, are optional. Values of
256 and above are invalid.
192.168.254.254Typical IP Address:
Octets
Figure 35. IP Address and Octets
Subnet masks and octets
The subnet mask (figure 36, below) 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 zeros, up to 3 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 36. 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 37, below). The octets of each address are compared or not,
depending on the value in the related subnet mask octet.
• 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 37).
• 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 X in figure 37).
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 37. Comparing the IP Addresses
