Subnet Masks - ZyXEL Communications ZyXEL Vantage VSG-1000 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; 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.
IP Address
IP Address (Binary)
Subnet Mask
Subnet Mask (Binary)
IP Subnetting
Chart 7 "Natural" Masks
CLASS
A
B
C
Chart 8 Alternative Subnet Mask Notation
SUBNET MASK "1" BITS
/24
/25
/26
/27
/28
/29
/30
NETWORK NUMBER
192.168.1.
11000000.10101000.00000001.
255.255.255.
11111111.11111111.11111111.
VSG-1000 Vantage Service Gateway
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
0
00000000
0
00000000
HOST ID
O