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Ethernet Connection, cont’d
USP 507 • Ethernet Connection
B-6
Subnet masks and octets
The subnet mask (figure B-5) 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 B–5 — Subnet mask and octets
Determining whether devices are on the same subnet
To determine the subnet, the local device’s IP address is compared to the remote
device’s IP address (figure B-6 below). Each address’s octets 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’s address and the remote device’s IP address are unmasked.
Unmasked octets are compared (indicated by ? in figure B-6).
• If the subnet mask octet contains the value 0, the related octets of the local
device’s and remote device’s IP addresses are masked.
Masked octets are not compared (indicated by X in figure B-6).
If the unmasked octets of the two IP addresses match (indicated by = in figure B-6,
example 1), the two addresses are on the same subnet.
If the two unmasked fields do not match (indicated by an unequal sign in
figure B-6, 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 B–6 — Comparing the IP addresses