ZyXEL Communications SBG3500-N000 User Manual page 357

The following figure shows an example IP address in which the first three octets (192.168.1) are
the network number, and the fourth octet (16) is the host ID.
Figure 197 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 128 Subnet Masks
IP Address (Binary)
Subnet Mask (Binary)
Network Number
Host ID
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.
SBG3500-N000 User's Guide
2ND
1ST OCTET:
OCTET:
(192)
(168)
11000000 10101000
11111111 11111111
11000000 10101000
Appendix B IP Addresses and Subnetting
3RD
4TH OCTET
OCTET:
(2)
(1)
00000001 00000010
11111111 00000000
00000001
00000010
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