Extron electronics ISM 824 - Subnet Masks and Octets; Determining Subnet Membership

A-7
ISM 824 Integrated Scaling Multiswitcher • Reference Information
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 A-6). Each address’s octets 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 local
device’s address and the remote device’s IP address are unmasked.
Unmasked octets are compared (indicated by ? in figure A-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 A-6).
If the unmasked octets of the two IP addresses match (indicated by = in figure A-6)
(example 1), the two addresses are on the same subnet.
If the two unmasked fields do not match (indicated by ≠ in figure A-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 A-6 — Comparing the IP addresses