Appendix B IP Addresses and Subnetting
Figure 51 Network Number and Host ID
How much of the IP address is the network number and how much is the host ID varies according to the subnet mask.
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). The term “subnet” is short for “sub- network”.
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.
The following example shows a subnet mask identifying the network number (in bold text) and host ID of an IP address (192.168.1.2 in decimal).
Table 14 IP Address Network Number and Host ID Example
| 1ST OCTET: | 2ND | 3RD | 4TH OCTET |
| OCTET: | OCTET: | ||
| (192) | (168) | (1) | (2) |
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IP Address (Binary) | 11000000 | 10101000 | 00000001 | 00000010 |
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Subnet Mask (Binary) | 11111111 | 11111111 | 11111111 | 00000000 |
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Network Number | 11000000 | 10101000 | 00000001 |
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Host ID |
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| 00000010 |
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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.
Subnet masks can be referred to by the size of the network number part (the bits with a “1” value). For example, an
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