Subnetting; Example: Two Subnets; Table 87 Alternative Subnet Mask Notation; Table 88 Two Subnets Example - ZyXEL Communications PRESTIGE 2602R Series User Manual

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.

Table 87 Alternative Subnet Mask Notation

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.

Table 88 Two Subnets Example

IP Address
IP Address (Binary)
Subnet Mask
Subnet Mask (Binary)
Appendix C IP Subnetting
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.
Prestige 2602R Series User's Guide
LAST OCTET BIT VALUE
0000 0000
1000 0000
1100 0000
1110 0000
1111 0000
1111 1000
1111 1100
HOST ID
0
00000000
0
00000000
294