ZyXEL Communications Prestige 662HW Series, User Manual

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Prestige 662HW Series User’s Guide
B-2 IP Subnetting
Chart B-2 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
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; each bit of the mask
corresponds to a bit of the IP address. 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 as IP addresses are. The “natural” masks
for class A, B and C IP addresses are as follows.
Chart B-3 “Natural” Masks
CLASS NATURAL MASK
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0
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.
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.
Chart B-4 Alternative Subnet Mask Notation
SUBNET MASK IP ADDRESS SUBNET MASK “1” BITS LAST OCTET BIT VALUE
255.255.255.0 /24 0000 0000
255.255.255.128 /25 1000 0000
255.255.255.192 /26 1100 0000
255.255.255.224 /27 1110 0000