ZyXEL Communications ZyXEL ZyWALL 2WG - Manual (Page 665)

Appendix E IP Addresses and Subnetting
ZyWALL 2WG User’s Guide
665
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.
The first mask shown is the class “C” natural mask. Normally if no mask is specified it is
understood that the natural mask is being used.
Table 255 “Natural” Masks
CLASS NATURAL MASK
A 255.0.0.0
B 255.255.0.0
C 255.255.255.0
Table 256 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
255.255.255.240 /28 1111 0000 240
255.255.255.248 /29 1111 1000 248
255.255.255.252 /30 1111 1100 252