ZyWALL 5 User’s Guide
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Appendix C IP Subnetting
Since the first octet of a class “A” IP address must contain a “0”, the first octet of a class “A”
address can have a value of 0 to 127.
Similarly the first octet of a class “B” must begin with “10”, therefore the first octet of a class
“B” address has a valid range of 128 to 191. The first octet of a class “C” address begins with
“110”, and therefore has a range of 192 to 223.
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 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 as IP addresses are. 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 left most 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.
Table 13
Allowed IP Address Range By Class
CLASS
ALLOWED RANGE OF FIRST OCTET
(BINARY)
ALLOWED RANGE OF FIRST OCTET
(DECIMAL)
Class A
0
0000000 to
0
1111111
0 to 127
Class B
10
000000 to
10
111111
128 to 191
Class C
110
00000 to
110
11111
192 to 223
Class D
1110
0000 to
1110
1111
224 to 239
Table 14
“Natural” Masks
CLASS
NATURAL MASK
A
255.0.0.0
B
255.255.0.0
C
255.255.255.0
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Страница 96: ...ZyWALL 5 User s Guide 94 Chapter 4 LAN Screens ...
Страница 102: ...ZyWALL 5 User s Guide 100 Chapter 5 Bridge Screens ...
Страница 130: ...ZyWALL 5 User s Guide 128 Chapter 7 WAN Screens Figure 45 WAN PPPoE Encapsulation ...
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Страница 174: ...ZyWALL 5 User s Guide 172 Chapter 10 Firewall Screens Figure 68 Creating Editing A Firewall Rule ...
Страница 179: ...ZyWALL 5 User s Guide Chapter 10 Firewall Screens 177 Figure 73 My Service Rule Configuration ...
Страница 220: ...ZyWALL 5 User s Guide 218 Chapter 13 Introduction to IPSec ...
Страница 238: ...ZyWALL 5 User s Guide 236 Chapter 14 VPN Screens Figure 110 VPN Rules IKE Network Policy Edit ...
Страница 261: ...ZyWALL 5 User s Guide Chapter 15 Certificates 259 Figure 122 My Certificate Details ...
Страница 280: ...ZyWALL 5 User s Guide 278 Chapter 16 Authentication Server Figure 133 Local User Database ...
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Страница 352: ...ZyWALL 5 User s Guide 350 Chapter 21 Remote Management ...
Страница 366: ...ZyWALL 5 User s Guide 364 Chapter 23 Logs Screens Figure 188 Log Settings ...
Страница 389: ...ZyWALL 5 User s Guide Chapter 24 Maintenance 387 Figure 210 Restart Screen ...
Страница 390: ...ZyWALL 5 User s Guide 388 Chapter 24 Maintenance ...
Страница 416: ...ZyWALL 5 User s Guide 414 Chapter 27 WAN and Dial Backup Setup ...
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Страница 536: ...ZyWALL 5 User s Guide 534 Chapter 42 Troubleshooting Figure 343 Java Sun ...
Страница 564: ...ZyWALL 5 User s Guide 562 Appendix C IP Subnetting ...
Страница 570: ...ZyWALL 5 User s Guide 568 Appendix E PPTP ...
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