P-2602H(W)(L)-DxA Series User’s Guide
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. 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 like IP addresses. The “natural” masks for class A, B and C IP addresses are as follows.
Table 135 “Natural” Masks
CLASS
A
B
C
NATURAL MASK
255.0.0.0
255.255.0.0
255.255.255.0
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 leftmost 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 136 Alternative Subnet Mask Notation
SUBNET MASK | SUBNET MASK “1” BITS | LAST OCTET BIT VALUE | DECIMAL |
|
|
|
|
255.255.255.0 | /24 | 0000 0000 | 0 |
|
|
|
|
255.255.255.128 | /25 | 1000 0000 | 128 |
|
|
|
|
255.255.255.192 | /26 | 1100 0000 | 192 |
|
|
|
|
255.255.255.224 | /27 | 1110 0000 | 224 |
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|
|
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Appendix D IP Addresses and Subnetting | 355 |
