P-662H/HW-D Series User’s Guide
Appendix E IP Addresses and Subnetting
391
Subnet Masks
A subnet mask is used to determine which bits are part of the network number, and which bits
are part of the host ID (using a logical AND operation).
A subnet mask has 32 bits. If a bit in the subnet mask is a “1” then the corresponding bit in the
IP address is part of the network number. If a bit in the subnet mask is “0” then the
corresponding bit in the IP address is part of the host ID.
Subnet masks are expressed in dotted decimal notation just like IP addresses. The “natural”
masks for class A, B and C IP addresses are as follows.
Subnetting
With subnetting, the class arrangement of an IP address is ignored. For example, a class C
address no longer has to have 24 bits of network number and 8 bits of host ID. With
subnetting, some of the host ID bits are converted into network number bits.
By convention, subnet masks always consist of a continuous sequence of ones beginning from
the leftmost bit of the mask, followed by a continuous sequence of zeros, for a total number of
32 bits.
Since the mask is always a continuous number of ones beginning from the left, followed by a
continuous number of zeros for the remainder of the 32 bit mask, you can simply specify the
number of ones instead of writing the value of each octet. This is usually specified by writing
a “/” followed by the number of bits in the mask after the address.
For example, 192.1.1.0 /25 is equivalent to saying 192.1.1.0 with mask 255.255.255.128.
The following table shows all possible subnet masks for a class “C” address using both
notations.
Table 151
“Natural” Masks
CLASS
NATURAL MASK
A
255.0.0.0
B
255.255.0.0
C
255.255.255.0
Table 152
Alternative Subnet Mask Notation
SUBNET MASK
SUBNET MASK “1” BITS
LAST OCTET BIT VALUE
DECIMAL
255.255.255.0
/24
0000 0000
0
255.255.255.128
/25
1000 0000
128
255.255.255.192
/26
1100 0000
192
255.255.255.224
/27
1110 0000
224
Summary of Contents for 802.11g ADSL 2+ 4-Port Security Gateway HW-D Series
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Page 64: ...P 662H HW D Series User s Guide 64 Chapter 2 Introducing the Web Configurator ...
Page 84: ...P 662H HW D Series User s Guide 84 Chapter 4 Bandwidth Management Wizard ...
Page 108: ...P 662H HW D Series User s Guide 108 Chapter 5 WAN Setup ...
Page 122: ...P 662H HW D Series User s Guide 122 Chapter 6 LAN Setup ...
Page 156: ...P 662H HW D Series User s Guide 156 Chapter 8 DMZ ...
Page 202: ...P 662H HW D Series User s Guide 202 Chapter 11 Firewall Configuration ...
Page 210: ...P 662H HW D Series User s Guide 210 Chapter 12 Anti Virus Packet Scan ...
Page 214: ...P 662H HW D Series User s Guide 214 Chapter 13 Content Filtering ...
Page 232: ...P 662H HW D Series User s Guide 232 Chapter 14 Content Access Control ...
Page 238: ...P 662H HW D Series User s Guide 238 Chapter 15 Introduction to IPSec ...
Page 273: ...P 662H HW D Series User s Guide Chapter 17 Certificates 273 Figure 144 My Certificate Details ...
Page 292: ...P 662H HW D Series User s Guide 292 Chapter 18 Static Route ...
Page 304: ...P 662H HW D Series User s Guide 304 Chapter 19 Bandwidth Management ...
Page 308: ...P 662H HW D Series User s Guide 308 Chapter 20 Dynamic DNS Setup ...
Page 332: ...P 662H HW D Series User s Guide 332 Chapter 22 Universal Plug and Play UPnP ...
Page 338: ...P 662H HW D Series User s Guide 338 Chapter 23 System ...
Page 344: ...P 662H HW D Series User s Guide 344 Chapter 24 Logs ...
Page 350: ...P 662H HW D Series User s Guide 350 Chapter 25 Tools ...
Page 364: ...P 662H HW D Series User s Guide 364 Chapter 27 Troubleshooting ...
Page 368: ...P 662H HW D Series User s Guide 368 Product Specifications ...
Page 372: ...P 662H HW D Series User s Guide 372 Appendix C Wall mounting Instructions ...
Page 408: ...P 662H HW D Series User s Guide 408 Appendix F Wireless LANs ...
Page 420: ...P 662H HW D Series User s Guide 420 Appendix H Command Interpreter ...
Page 436: ...P 662H HW D Series User s Guide 436 Appendix L NetBIOS Filter Commands ...
Page 462: ...P 662H HW D Series User s Guide 462 Appendix M Internal SPTGEN ...
Page 484: ...P 662H HW D Series User s Guide 484 Appendix P Triangle Route ...