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MultiPort/LT User Reference
Page 11
Introduction to Network Addressing
1.5.5.
Using Subnetmask
The address class determines the length of the external network number.
❏
Example
ADDRESS 220.72.117.26
This is a Class C address - and therefore the external network
number is 220.72.117 (first 24 bits).
The subnetmask must
always
decode the external network number as
belonging to the network number.
❏
Example
ADDRESS
220.72.117.26
SUBNETMASK
255.255.255.240
This is a valid subnetmask for a Class C address.
The subnetmask must
never
decode (part of) the external network number as
belonging to the host number.
❏
Example
ADDRESS
220.72.117.26
SUBNETMASK
255.255.240.0
This is an illegal IP address and subnetmask combination. Part of the
external network number is decoded as belonging to the host number.
1.5.6.
Multiple Networks
Organizations having multiple internal networks use the external network
number to give the appearance of a single network to the outside world.
Refer to Figure 1-9.