Subnet Masks - ZyXEL Communications P-660HN-51 User Manual

Figure 171 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 104 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.
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.
Appendix B IP Addresses and Subnetting
1ST OCTET: 2ND OCTET:
(192) (168)
11000000 10101000
11111111 11111111
11000000 10101000
P-660HN-51 User's Guide
264
3RD OCTET: 4TH OCTET
(1) (2)
00000001 00000010
11111111 00000000
00000001
00000010