ZyXEL Communications NBG-418N v2, User Manual

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Appendix A IP Addresses and Subnetting
NBG-418N v2 User’s Guide
149
Figure 111
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 59
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