ZyXEL Communications Vantage CNM 2.0 User Manual page 226

Vantage CNM 2.0
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
255.255.255.248
255.255.255.252
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.
Example: Two Subnets
As an example, you have a class "C" address 192.168.1.0 with subnet mask of 255.255.255.0.
P
"Natural" Masks
CLASS
A
B
C
Alternative Subnet Mask Notation
SUBNET MASK "1" BITS
/24
/25
/26
/27
/28
/29
/30
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
1111 1000
1111 1100
IP Subnetting