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 OfficeConnect Remote can be configured to use subnet masking to enable ISDN Autocalls to be made on groups of IP addresses or on specific IP addresses.
Routing IP and IPX A-13
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.
This is achieved by the addition of a /xx number at the end of the configured address. For example:
193.123.123.123/32
The /32 appended to the IP number indicates that all four of the bytes are used and must have valid entries.
A /24 mask, for example:
193.123.123.0/24
means that only the first three bytes are to be recognized and used, and the last byte can be ignored. In this case, any IP address appearing on the locally connected LAN in the range 193.123.123.0 to 193.123.123.255 will cause an autocall to be made to the destination network who’s name is associated with that number in the ISDN Autocall table. The name is then looked up in the ISDN Numbers table and a call made to the ISDN number which has been assigned to that name.