Appendix B IP Addresses and Subnetting
Subnet masks are expressed in dotted decimal notation just like IP addresses. The following examples show the binary and decimal notation for
Table 15 Subnet Masks
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| 1ST | 2ND | 3RD | 4TH OCTET | DECIMAL |
| OCTET | OCTET | OCTET |
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11111111 | 00000000 | 00000000 | 00000000 | 255.0.0.0 | |
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11111111 | 11111111 | 00000000 | 00000000 | 255.255.0.0 | |
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11111111 | 11111111 | 11111111 | 00000000 | 255.255.255.0 | |
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11111111 | 11111111 | 11111111 | 11111000 | 255.255.255.248 | |
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Network Size
The size of the network number determines the maximum number of possible hosts you can have on your network. The larger the number of network number bits, the smaller the number of remaining host ID bits.
An IP address with host IDs of all zeros is the IP address of the network (192.168.1.0 with a
As these two IP addresses cannot be used for individual hosts, calculate the maximum number of possible hosts in a network as follows:
Table 16 Maximum Host Numbers
SUBNET MASK | HOST ID SIZE |
| MAXIMUM NUMBER OF HOSTS | |
8 bits | 255.0.0.0 | 24 bits | 224 – 2 | 16777214 |
16 bits | 255.255.0.0 | 16 bits | 216 – 2 | 65534 |
24 bits | 255.255.255.0 | 8 bits | 28 – 2 | 254 |
29 bits | 255.255.255.248 | 3 bits | 23 – 2 | 6 |
Notation
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 subnet mask 255.255.255.128.
The following table shows some possible subnet masks using both notations.
Table 17 Alternative Subnet Mask Notation
SUBNET MASK | ALTERNATIVE | LAST OCTET | LAST OCTET | |
NOTATION | (BINARY) | (DECIMAL) | ||
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255.255.255.0 | /24 | 0000 0000 | 0 | |
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255.255.255.128 | /25 | 1000 0000 | 128 | |
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| 65 |
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