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Appendix B IP Addresses and Subnetting
STB-1001S2 User’s Guide
64
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).
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 “8-bit mask” means that the first 8 bits of the mask are ones and the
remaining 24 bits are zeroes.
Table 14 IP Address Network Number and Host ID Example
1ST OCTET:
(192)
2ND
OCTET:
(168)
3RD
OCTET:
(1)
4TH OCTET
(2)
IP Address (Binary) 11000000 10101000 00000001 00000010
Subnet Mask (Binary) 11111111 11111111 11111111 00000000
Network Number 11000000 10101000 00000001
Host ID 00000010