ZyXEL P-871H User Guide - Page 90

Example: Two Subnets

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P-871H Series User's Guide 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 26 Alternative Subnet Mask Notation SUBNET MASK IP ADDRESS SUBNET MASK "1" BITS 255.255.255.0 /24 255.255.255.128 /25 255.255.255.192 /26 255.255.255.224 /27 255.255.255.240 /28 255.255.255.248 /29 255.255.255.252 /30 LAST OCTET BIT VALUE 0000 0000 1000 0000 1100 0000 1110 0000 1111 0000 1111 1000 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. Table 27 Two Subnets Example IP Address IP Address (Binary) Subnet Mask Subnet Mask (Binary) NETWORK NUMBER 192.168.1. 11000000.10101000.00000001. 255.255.255. 11111111.11111111.11111111. HOST ID 0 00000000 0 00000000 The first three octets of the address make up the network number (class "C"). You want to have two separate networks. Divide the network 192.168.1.0 into two separate subnets by converting one of the host ID bits of the IP address to a network number bit. The "borrowed" host ID bit can be either "0" or "1" thus giving two subnets; 192.168.1.0 with mask 255.255.255.128 and 192.168.1.128 with mask 255.255.255.128. IP Subnetting 89

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P-871H Series User’s Guide
IP Subnetting
89
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.
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.
The first three octets of the address make up the network number (class “C”). You want to
have two separate networks.
Divide the network 192.168.1.0 into two separate subnets by converting one of the host ID bits
of the IP address to a network number bit. The “borrowed” host ID bit can be either “0” or “1”
thus giving two subnets; 192.168.1.0 with mask 255.255.255.128 and 192.168.1.128 with
mask 255.255.255.128.
Table 26
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
Table 27
Two Subnets Example
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