3Com OfficeConnect Remote 510, User Manual

Page: 67 / 96

Routing IP and IPX A-13
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.
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.
Rc.bk : RCAPPA.FRM Page 13 Thursday, July 10, 1997 9:53 AM