ZyXEL Communications ZyWALL 5 User Manual page 496

ZyWALL 5 Internet Security Appliance
CLASS ALLOWED RANGE OF FIRST OCTET
Class A 00000000 to 01111111
10000000 to 10111111
Class B
Class C 11000000 to 11011111
Class D 11100000 to 11101111
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.
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.
SUBNET MASK IP ADDRESS
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
255.255.255.240
J-2
Chart J-2 Allowed IP Address Range By Class
(BINARY)
Chart J-3 "Natural" Masks
CLASS
A
B
C
Chart J-4 Alternative Subnet Mask Notation
SUBNET MASK "1" BITS
/24
/25
/26
/27
/28
ALLOWED RANGE OF FIRST OCTET
(DECIMAL)
0 to 127
128 to 191
192 to 223
224 to 239
NATURAL MASK
255.0.0.0
255.255.0.0
255.255.255.0
LAST OCTET BIT VALUE
0000 0000
1000 0000
1100 0000
1110 0000
1111 0000
IP Subnetting