P-2000W_V2 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 33 Alternative Subnet Mask Notation
SUBNET MASK IP ADDRESS | SUBNET MASK “1” BITS | LAST OCTET BIT VALUE |
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255.255.255.0 | /24 | 0000 0000 |
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255.255.255.128 | /25 | 1000 0000 |
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255.255.255.192 | /26 | 1100 0000 |
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255.255.255.224 | /27 | 1110 0000 |
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255.255.255.240 | /28 | 1111 0000 |
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255.255.255.248 | /29 | 1111 1000 |
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255.255.255.252 | /30 | 1111 1100 |
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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 34 Two Subnets Example
| NETWORK NUMBER | HOST ID |
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IP Address | 192.168.1. | 0 |
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IP Address (Binary) | 11000000.10101000.00000001. | 00000000 |
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Subnet Mask | 255.255.255. | 0 |
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Subnet Mask (Binary) | 11111111.11111111.11111111. | 00000000 |
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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.
| Appendix B IP Subnetting | 113 |
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