Extron electronic USP 507 manual Subnet masks and octets, Ethernet Connection, cont’d

Ethernet Connection, cont’d

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 indicates that this octet will be compared between two IP addresses.

0 indicates that this octet will not be compared between two IP addresses.

Typical Subnet Mask: 255.255.0.0

Octets

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.

	Example 1	Example 2	Example 3
Local IP Address:	192.168.254.254	192.168.254.254	192.168.254.254
Subnet Mask:	255.255.0.0 (?.?.X.X)	255.255.0.0 (?.?.X.X)	255.255.0.0 (?.?.X.X)
Remote IP Address:	192.168.2.25	190.190.2.25	192.190.2.25
Match?:	=.=.X.X — Match	≠.≠.X.X — No match	=.≠.X.X — No match
	(Same subnet)	(Different subnet)	(Different subnet)

Figure B–6 — Comparing the IP addresses

B-6 USP 507 • Ethernet Connection