Subnet Masking - 3Com AccessBuilder 500 User Manual

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PPENDIX RIDGING AND OUTING
Using of any of the address classes in a private TCP/IP network is not a
problem, providing that connections outside of that private network
to external public or private TCP/IP networks are never needed. If a
private IP addressing number scheme is established within a private
corporate network, connections out of that network to external public
or other private TCP/IP networks, can be achieved via a computer
which has software which enables it to act as an IP gateway. These
devices, if configured correctly, provide the IP numbering/address
translation between the two networks.

Subnet Masking

Subnet Masking is a mechanism which can be enabled in computer
and communications equipment which tells the equipment and the
network, which parts of the IP address are to be used as the Network
identifier and which are the Host identifier.
A subnet mask consists of a similar field structure to that of the IP
address (123.123.123.123). For example:
255.255.0.0
This means that the first two three digit bytes of the IP address (the
fields masked by - 255.255) are to be recognized and used as the
Network address, and the last two bytes (those set to .0.0) are to be
used to identify the Host address.
An alternative way of expressing a subnet mask is a single number
indicating how many bits of the IP address are to be used for the
network address. For example 255.255.0.0 can be expressed as 16 while
255.255.255.192 can be expressed as 24.
The AccessBuilder 500 can be configured to use subnet masking to
enable ISDN Autocalls to be made on groups of IP addresses or on
specific IP addresses.
Normally, a subnet mask would be set so that any IP address, in a range
of hosts on a destination LAN which are detected on the unit's locally
connected LAN port, causes an ISDN Autocall to be made out to that
destination.