Extron electronic DXP DVI PRO manual Subnet masks and octets

Subnet masks and octets

The subnet mask (figure 122) 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 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 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 122. 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 123, 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 123).

•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 n in figure 123).

If the unmasked octets of the two IP addresses match (indicated by = in example 1 of figure 123), the two addresses are on the same subnet.

If the two unmasked fields do not match (indicated by an unequal sign [≠] in figure 123, examples 2 and 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
Match?:		= . = .X.X —			≠ . ≠ .X.X — No match	= . ≠ .X.X — No match
	(Same subnet)			(Different subnet)		(Different subnet)

Figure 123. Comparing the IP Addresses

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