Subnet Masks; Subnetting; Table 98 "Natural" Masks; Table 99 Alternative Subnet Mask Notation - ZyXEL Communications VES-1616F-44 User Manual

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.
Table 98
CLASS
A
B
C

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 99 Alternative Subnet Mask Notation

SUBNET MASK
255.255.255.0
255.255.255.128
255.255.255.192
255.255.255.224
Appendix B IP Addresses and Subnetting
"Natural" Masks
NATURAL MASK
255.0.0.0
255.255.0.0
255.255.255.0
SUBNET MASK "1" BITS
/24
/25
/26
/27
VES-1616F/1624F-44 User's Guide
LAST OCTET BIT VALUE
0000 0000
1000 0000
1100 0000
1110 0000
DECIMAL
0
128
192
224
283