ZyWALL 10~100 Series Internet Security Gateway

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.

Chart 8-4 Alternative Subnet Mask Notation

SUBNET MASK IP ADDRESS

SUBNET MASK “1” BITS

LAST OCTET BIT VALUE

 

 

 

255.255.255.0

/24

0000 0000

 

 

 

255.255.255.128

/25

1000 0000

 

 

 

255.255.255.192

/26

1100 0000

 

 

 

255.255.255.224

/27

1110 0000

 

 

 

255.255.255.240

/28

1111 0000

 

 

 

255.255.255.248

/29

1111 1000

 

 

 

255.255.255.252

/30

1111 1100

 

 

 

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.

 

NETWORK NUMBER

HOST ID

 

 

 

IP Address

192.168.1.

0

 

 

 

IP Address (Binary)

11000000.10101000.00000001.

00000000

 

 

 

Subnet Mask

255.255.255.

0

 

 

 

Subnet Mask (Binary)

11111111.11111111.11111111.

00000000

 

 

 

IP Subnetting

8-3