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What is a network?
141
reserved for other purposes). By contrast, a Class A network’s IP address allocates
eight bits for the network number and 24 bits for the local address, allowing for a
possible combination of 16,777,214 (2
24
-2) local addresses.
To return to the postal analogy, just as a street address refers to the set location or
residence itself and not to the occupant, who may change over time, an IP address
refers to a device’s location and not the device itself. Thus, for example, if a PC is
relocated to another area of the same network, it does not retain the same IP
address.
What is a Subnet Mask?
A subnet mask is a pattern of 32 binary digits (ones or zeroes) which is transmitted
alongside an IP address to instruct certain routing devices (namely, internal routers
within private organizations, as opposed to public, Internet routers) to read or
interpret the IP address in an alternate way.
Specifically, a subnet mask instructs a routing device to take the
host number
portion of the IP address and further subdivide it into two smaller segments: a
subnet number a
nd an even shorter
host number
. Where this subdivision occurs
depends upon the pattern of ones vs. zeros in the subnet mask. That is, within the
string of the subnet mask’s 32 bits, the point at which the ones end and the
zeroes begin is the point at which the subdivision is made. The resulting
subnet
number
is then added to the end of the original
network number
, such that a
new, longer network number is temporarily derived.
The mechanism of a subnet mask is analogous to a colored lens used in an
optician’s test to blot out one half of a two-toned color pattern in order to reveal a
second, sub-pattern. Just as the colored lense is used to “mask” all of one color,
the string of ones in a subnet mask is used to mask any bit in the IP address that
corresponds to a one in the subnet mask, when the address and mask are
matched up against one another. This idea is illustrated in Figure 70, which shows
a subnet mask in a Class C network.
Figure 70
Subnet Masking
Subnet Mask Applied to IP Address in a Class C Network
11 0 0 0 0 0 0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0 0 0 0 0 1
11 0 0 0 0 0 0 . 1 0 1 0 1 0 0 0 . 0 0 0 0 0 0 0 1 . 0 0 0
0 0 0 0 1
11111111 . 11111111 . 11111111 . 111 0 0 0 0 0
Network
Number
(24 bits)
Host
Number
(8 bits)
(New)
Network
Number
(27 bits)
(New)
Host
Number
(5 bits)
Subnet
Number
(3 bits)
IP Address
(as read by external router)
Subnet Mask
IP Address
(as read by internal router,
through "lense" of Subnet Mask)
Bits left unmasked
by zeroes
are read as part of
(New) Host Number
Bits "masked"
by ones
are read as part of
(New) Network
Number
Summary of Contents for U.S. Robotics 56K Voice
Page 10: ...GLOSSARY 3COM CORPORATION LIMITED LIFETIME WARRANTY REGULATORY AND APROVAL INFORMATION...
Page 30: ...30 CHAPTER 2 DUAL 56K LAN MODEM FUNCTIONALITY DESCRIPTION...
Page 46: ...46 CHAPTER 4 INSTALLING THE DUAL 56K LAN MODEM...
Page 58: ...58 CHAPTER 5 CONFIGURING THE DUAL 56K LAN MODEM FOR INTERNET ACCESS...
Page 124: ...124 CHAPTER 9 PLACING RECEIVING AND DISCONNECTING CALLS...
Page 136: ...136 CHAPTER 10 TROUBLESHOOTING AND MAINTENANCE...
Page 146: ...146 APPENDIX A NETWORKING PRIMER...
Page 154: ...154 APPENDIX D FACTORY DEFAULTS...
Page 156: ...156 APPENDIX E SPECIFICATIONS...
Page 166: ......