Subnet Masks - ZyXEL Communications ZyXEL Prestige P-870HW-51a v2 User Manual

Appendix C IP Addresses and Subnetting
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 150 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 59 IP Address Network Number and Host ID Example
IP Address (Binary)
Subnet Mask (Binary)
Network Number
Host ID
236
1ST 2ND
OCTET: OCTET:
(192) (168)
11000000 10101000
11111111 11111111 11111111 00000000
11000000 10101000 00000001
3RD 4TH
OCTET: OCTET
(1) (2)
00000001 00000010
00000010
P-870HW-51a v2 User's Guide